Right, just thought I'd make a thread for everyone to post up moments where the players have defied all the expectations of the GM or even themselves!
Moments where the players have had their backs to the wall, a knife at their throat and a shotgun at their head, but they've managed to escape and tell the tale.
My campaign is still in it's infancy at the mo, but the players have just crash landed on Hesh in the Lathe Worlds where there's a bit of a tech-schism and conflict going on, I'll let you know how it all goes

My players have been sent out into the halo stars to investigate the activities of a Rogue Trader thought to be dealing in xeno artifacts. Whilst undercover on an outpost they get roped into solving a murder which boils down to "who fired the shuriken pistol, the rogue trader with a set of captured pistols or the mysterious eldar outcast also on the station?"

Naturally they split the party.

One group heads off to the rogue trader and starts asking questions and getting answers, this half of the party are smart and ask a lot of revealing questions which get them revealing answers, a little more digging and they could have solved half the campaign then and there.

Meanwhile the second group hit upon a somewhat less than orthodox plan: "If we attempt to assassinate the Eldar then he'll fire on us and we can match the shurikens against the ones in the body of the murder victim." I was prepared for a couple of lines of inquiry, but not for this. Fortunately I'd statted this guy, I might not have forseen this but I was able improvise as they agreed to meet the outcast in a classy bar on the outpost and they started coming up with an ambush, I prepared for the outcast and a couple of bodyguard to show up. It was at this point that group B declared that they were going to the meet without calling for backup or even informing group A of their plan. I tried to hint that this was not a good idea and asked them if they had a backup plan in case things went wrong? Their reponse: "If things go wrong we'll just run away from the Eldar"

To summerise:

Plan A: Goad the xeno mercenaries into a firefight in the middle of an unfriendly bar a long, long way from imperial space without backup or nearby support

Plan B: Run away. From the Eldar.

That was the point at which I went

The bit where they survived only by burning fate points, one character having his arm severed off and another one his leg almost mangled beyond repair before they reached a service elevator and escaped, that was the point I stopped.

New moment.

My players have just had a run in with a Sector spanning crime Organisation, known to the Acolytes and others as "The Syndicate". Our Arbitrator (played by me) immediatley backed out of the room, before panickingly telling the Acolytes they're part of the Syndicate, their Infamy, Omnipotence etc etc. Our Psyker, one Ivan Blackheart decides:
"Well, let's see what they know about the Spook Trade" (which we're investigating). He promptly rushes back into the bar, blows 2 of the members heads open, grabs the other and heads back out into the street, shouting:
"Adeptus Arbites, don't interfere!!!!" to any onlookers.

Seriouslly....

(Spoiler: Shades on Twilight)

The moritat assassin in my group challenged Akirvas the Dark Eldar from Shades on Twilight to a duel... Akirvas smiled and accepted. So naturally when the moritat charged, he was completely surprised when he got blitzed by the two mutated pets, followed up by a nice spritz from the destructor...

He survived though. Fate points come in handy apparently.

One of my group's psykers (we have 2), who is a Templar Secundus of the Scholastia Psykana, was the first to encounter Akirvas.

Akirvas got cut in half vertically, from head to crotch, from a single sword blow... before he even had the chance to blow the whistle.

Posing as hab-workers my group tries to find a friendly game to schmooze some local info. Do to horrible dice rolling they set themselves up to get jumped and relieved of their thrones. At most they have is a blade apiece (and an overconfident feral psyker), and are unarmoured.

In the maze of corridors out step some local scum and point guns at the offworlders- caught with their work coveralls down. Short story short the psyker tries something clever but roles a 9 and all the lights flicker and the hivers guns all jam, since the pc psyker rolled a huge area of effect. The paniced acolytes started brawling and stabbing chasing off their ambushers and capturing one to interrogate. It went from"Why didnt YOU bring your laspistol/chainsword/flamer!!!???" to "this was the information we were looking for" and the psyker is trying to pass off to the group that jamming all the guns pointing at them was his intention. Fun group to GM.

My group was investigating a suspected case of Tech-Heresy on a non-descript hive world and had finally tracked down the dissident tech-priests to an abandoned hive. After a few bad roles with my tech-priest trying to get the machine spirits to cooperate, we arrived outside the manufactorium. The plan we came up with, one I was certain wasn't going to work, was for me to sabatoge the power supply to the factory while the rest of the party ambushed the hereteks who came out to fix it. While they were interrogating the heretek pair, the party's cleric and I (after the cleric had "borrowed" what I assume were the holy robes of the Adeptus Mechanicus off one of the hereteks) were to make it look like we were the guys the heretics sent out and the machine was being particularly contrary. I don't know about you, but I could think of at least three ways that plan could have gone wrong.

Supposedly, the GM ruled that I manage to make a convincing enough act that the cleric could follow along without difficulty while our Arbite fumbled his interrogation by forgetting about the mechadedrites both hereteks had... Keep in mind that the cleric was almost completely unaugmented and I was still rank one on the Tech-Priest career path, so apparently despite lacking the nearly iconic "third arm" both of us managed to convince the guys in the factory long enough to get the information we needed. And then the real fun began... By the time the final showdown had ended, I had been thrown into a wall from a laser blast big enough to have come out of a ship battery (don't ask,) the enemy psyker had thankfully failed most of his rolls long enough for our assassin and arbite to mow him down, and the cleric Robert Marely (I wish I was kidding) had graduated to MVP by gunning down the head heretek with his auto pistol.

The moment came with the realization that we had all (barely) survived something that should have killed half of us... without anyone burning a single fate point .

My friends group had a LoL moment just last week. They had been tasked with infiltrating a heretic compound and listening in on a meeting. They're group consisted of an assasin, 2 guard, a psyker, and a scum. While trying to climb into a vent one guard player fell THREE TIMES alerting quite a large group of heretics. The group unanimosly decided to GTFO. They got seperated and decided they would meet up at a large cloning chamber nearby and figure out how to get out there. The psyker and a guard ended up running into a little girl, who proceeded to levitate and make **** explode. Demonhost anyone . The psyker tries to compel some nearby heretics to distract the demonhost and fails THREE tests, succesfully turning himself into a demonhost. The "psyker" demon host proceeded to tear open the little girl with his bare hands and melt his guard team mate with a lightning bolt. The scum and assasin tried to flee the room only for the scum to have his head explode and the assasin also get melted to the ground. The last guard player tried to climb into a cloning tube to try and clone and army of herself. The demonhost turned off the cloning console and proceeded to walk up to the glass tube and smile at the guard as she drowned. Most epic wipe ever!

We had an moment in the last campaign I ran. It was set in an altered version of the Cirian Auxilla of the Cirian Legacy campaign book for inquisitor in which the mechanicus governing body has been overthrown by the miner's union. The PC's were trying to infiltrate an area of the labs which had been converted into a make-shift, over-charging hospital by a group of fiscal-minded rebels in order to administer medical aid to one of their comrades. The area was highly fortified, with 3 armed guards at each entrance and heavy stubber emplacements.

The cleric of the group decided the best way to get into the hospital would be to directly confront the security. At this point I started to panic- he was armed only with a broken chainsword (he tried to attack a thick metal floor with it for some reason) and a knife. He proceeded to take out a las-pack (he just happened to still have this in his inventory from when he threw away all of his ranged weapons) and stab it with his knife in close proximity to one of the guards, hoping it would produce an explosion. As I was feeling kind, I had it produce a small burst. Through ludicrously lucky dice rolls, this started a chain reaction in which the guard's shotgun ammo exploded, which in turn caused the other guards' ammo to explode and finally for the heavy stubber to be taken out of action. Massive damage was dealt to all of the enemies, as can be expected. How much damage did the cleric take? Just enough to knock him off his feet.

He stood up, strutted into the hospital, and threatened similar action if his party-member wasn't looked after!

SPOILER ALERT! If you haven't played The House of Dust and Ash yet, don't read any further! You have been warned!

We played The House of Dust and Ash for the first time, and things went down with just as much paranoia at the start, and then the **** really hit the fan towards the end. I played a Techpriest and he sort of "befriended" the academic Lanus Cisten during the adventure, and also decided to reveal the acolyt'es true mission and intention during the auction. Naturally Lanus was scared shitless and was about to break from hearing this, but my techpriest promised on pain of his honor that he would protect Lanus from harm during the whole affair, in exchange for his cooperation (namely the funds he had brought to the auction since our first plan was to try to buy the most coveted items and see how the other groups reacted and what bids they were willing to make). He gladly accepted of course.

The rest of the adventure ran it's course, and we escaped the crematoria at the end, and the only survivors from the incident were my cell of acolytes, along with Lanus who my Techpriest had successfully kept safe, and also the escaped Octavia Nail.

Once back on the airship, heading for safety, Octavia Nail took Lanus hostage and used him as a human shield. During the situation only my Techpriest and the cell's Black Priest of Maccabeus was present at the time. The situation seemed tense, and my Techpriest said:

-"I promised that I would insure that he did not come to any harm."

The Black Priest simply answered: -"I didn't..." then he shot a bolt round that pierced Lanus' body and hit Octavia, killing them both.

That was a real - moment.

And do remember, that most of the acolytes from that particular cell consider MY techpriest as being the heretical **** amongst them. And while he did betray them in the end and performed some rather heretical deeds, he was never as big a **** as the Black Priest was.

We're currently running through "Tattered Fates", just escaped the pit, and also the rabid cherub servitors and finally made our way into the control room with the lift. We lured two of the guards out of the room and beat them down (YAY we went clubbing! withareallybigclub) because my Adept wanted to question them. Mind you, my Adept has the persona and speech patterns of Dr. Daniel Schreber (Keifer Sutherland) from Dark City (For those of you who don't know, he's kinda meek, usually out of breath and rather excitable), so the whole group (Psyker, Tech Priest, IG, Scum, Assassin) did a collective "WHAAAAA..." when my Adept calmly stated

"Well this one will talk <gasp> that means the other is un<gasp>necessary. You <points at assassin> and you <points at Scum> take him down <gasp> the hall and dispose of him. Quietly please."

No one expected the Adept to be completely ruthless. I had fun that night <justasplannedBWAHAHAHAHAHAHA>

Ok this was from one of my old campaigns that i used to play. I was an adept with a group of IG fighting in the Sabbat Crusades (dont ask me why my old GM had a thing for the Gaunt's ghosts books) anyways i was the MEDIC i had no physical apptitude whatsoever just telling you this now.

our sniper in the group basicly Mad Larkin if you have read the books, was fighting another sniper with a mutated navigators eye. he killed him (it eventualy turned out to be a clone...so he came back about three times...) and the sniper goes "i want his eye...now..." the gm let me roll for it because earlier he had his eye blown out as well. Somehow it actualy worked and the sniper now had a heretic eye that he never got caught with. not my moment.

in the fight our commisar was grievly wounded and i had to stabalize him this however was not the plan the GM had in mind and through a mind boggling slew of rolls i managed to save him (he eventualy went on to use a rocket pack and a lot of explosives to take out a gaurd tower that we had to get rid of and kill himself...) also not my moment.

after the fight a chaos space marine bursted through the wall of the factorium we faught in (a world eater i think or a death eater i cant remember the chapter name) somehow there was a rocket launcher nearby that no one seemed to notice. my ADEPT picked it up loaded a rocket and rolled a 01! and with my favorite line ever "the doctor is in!" proceeded to blow the SM back to whatever hell spawned him...yeah ...that was a good day lol

Unlike many of these, this one isn't a combat one: while playing Rejoice For You Are True the party had to [minor spoiler warning] catch the eye of and impress a high-up from the Alabaster court. They reasoned that since everyone at Lady Borella's soiree was either a noble or a flunky they'd all be incredibly decadent, and so it would take something incredibly outre to impress them. Amusingly, three of the characters were disguised as either bodyguards (the Cleric and the Arbitrator) or clowns (the Psyker, a midget), and noted that the punch was glowing (a fashion statement). They then proceeded to dump a whole tureen of punch over the midget psychic clown, and juggle him between the two of them and a random bodyguard (from one of the other nobles who was persuaded through a quick charm test).

That's right, our party got into the Alabaster court by juggling a glowing, dripping, midget clown for a stuck-up bint in a dress covered in fairy lights...

Alasseo said:

We had an honest to the god Emperor Oompa Loompa look-a-like in one of our last games. He was an orange skin dyed midget psyker with green hair. Completely random character generation. The moment came when he tried to do a psychic power and rolled a 9. Perils of the Warp? Ghostly whispering and singing filling the room we were in with our names being used throughout the songs...

My party of five opened the steel doors to the Churgeon's medical theatre in the Alms House from the Edge of Darkness, only to find the Churgeon, 8 bodysnatchers, the scalpel familiar, and a homonculite there ... they quickly began closing the door again (three of them, the fourth was already in melee with the scalpel familiar that had burst out of the room, and the fifth with the homonculite). Just before it closed, the 8 bodysnatchers were detached from their racks and started pushing the door open again.

3 acolytes with Strengths of around 33 vs 8 bodysnatchers with augmented muscles .. before the door was swung open again and the tide of enemies could spill out, one acolyte retrieved two frag grenades from the belt of another and not tossed, no just dropped, or let them slide carefully from his hands to the other side of the door that was still ajar ...

KABOOM!

6 out of 8 bodysnatchers die!

The acolytes destroy the other enemies in a vicious fight, one burns a fate point, the fake enforcers set the Alms House on fire and the Churgeon disappears down her escape shaft with a full auto hit on her chest, leaving the players guessing to her fate....

My psyker has had 49 corruption for nearly three months now due to a deamon trying to enter his skull. That incident also gave him a Hatred for Tech-Priests. Enter the new guy, who makes a Tech-Priest. We are starting the campaign and the subject of my corruption came up again. He calmly tells everyone that he is pristine. Not a single dot of corruption or insanity. The Assassin says, "Too bad the Psyker can't corrupt you by reading your mind, but it only works if you read his." The Tech-Priest responds, "Nope, I am uncorruptable."

Half way through the session we figure we need some information rather badly. We nab a druken party goer and I attempt to slide into his head. The Tech-Priest should have seen this coming.... A set of rolls later and the Mass Possession has taken ahold of the party for 14 rounds, and when these devilish guests leave, they drop off a nice 44 corruption points as a farewell gift. Mister Pristine Tech-Priest went from no corruption to having 44 corruption, several malignancies, and a mutation!

On a side note, my psyker currently has 96 corruption now... Yeah, I am already working on a new character, because I don't see him surviving the next session. However, the GM said that he will become a recurring villian, and it will take four deaths to keep him down.

my Adept a complete non combat character pinning down the deamon at the end of illumination my god it was one hell of a roll. i actully described him as glowing gold with golden wings apearining on his back the full wrath of the emperor blessing him as he strugled to defeat the deamon it was epic!!