This thread is dedicated to those moments in game where the players come up with something so off the wall but at the same time is so clever that you are left with no choice but to run with it and see where it goes.
Ill start us off by telling a story about the last time i ran dark heresy with my group. It was the 2nd time we had ever played with our current group of acolytes and they turned what was going to be a standard blow the crap out of some guy and loot his base encounter into a close ally who modded their car and weapons.
Going into a little bit more detail, the acolytes were on their way back to the scintilla with their newly appointed "permanent loaned ship" when it came time to set it to auto pilot and hit the stasis pods until they landed. But when they woke from their stasis, their pods had been ejected from the ship somehow and their ship was gone along with the crew and its cargo. They had somehow ended up on an old mining world that had been sucked dry about 20 years ago. So after a bit of investigation they find out that they had crash landed their ship had been hauled off by raiders who control the southern territories of the planet. They figure that they might need a little extra fire power so they can get their ship back without being blown to bits in the process. So they head over the an old scrap yard and rig up an old jeep so they can load it up at the old military base and make their way to blow the raider base to hell.
At the military base however, was a tech priest who had taken refuge their and lives with the security bots and over the years had gone a little insane. He believes that the raiders are always watching him and are constantly trying to steal his research about the effects of grafting warp creatures flesh onto human bodies. He lets them in because they politely knocked on the front door instead of blowing it open with their bolters and they talk for a bit. When he finally becomes comfortable enough to take off his robe, the cleric notices and recognizes a piece of skin on his chest as that of a creature from the warp. He politely says that he can no longer be in his presence because its against the emperors will to allow such heresies to be in his presence.
This triggered his insanity switch and he seals the exit before the cleric can leave and he starts to spout off how they must be spies sent from the raiders to get his research, to which the group psyker and tech priest who were fascinated with this guys set up attempt to calm him down due to a fear of getting blown to bits and also a need for his equipment, which a fire fight might destroy.
It came down to when the insane tech priest gave them the ultimatum of "prove to me that you aren't a spy or you all die" to which the psyker responded with shooting himself in the hand with his las pistol and swearing on his blood that they weren't spies sent by the raiders. After a roll of a 1 on the players charm test and a double 0 on the tech priests opposed fellowship test, the group now has a new best friend who has an intimate knowledge of everything in the army base and a tech use skill that could rival Zeus's skill with a lightning bolt who modded their old jeep into what i call the A.W.M. The Acolyte War Machine! Fully equipped with a Las Gattling Gun in the front, a 360 degree bolter mounted on the back, Mechanical Arms, enough stummers to make it quieter than the most skilled ninja, tank treads, a hydrolic system that can launch them 10 meters into the air, and a customized driver seat that uses the group tech priest as the power source.
It should be noted that i didn't give them anything that they didn't already have access to after they were going to kill him, they just made a very useful friend in the process who just happens to know the complicated workings of the warp and how to create warp spawned flesh capable of an endless variety of uses.
All in all, i thought it was an amazing night worthy of going down in my groups hall of "Night that will go down in famy, not go down in infamy, just famy"