In a recent game a couple of large monsters got trapped in a corridor with a rubble token in it. I think the rubble came from a Crushing Block or similar trap. I couldn't find any special rules governing this... so I guess a single misplaced trap can really seal off a section of the map for large monsters. Am I right? It seems that it's even possible to make a quest unsolvable with careful placement of traps...

Yeah, unfortunately large monsters can be made near useless by a badly placed trap. Take the spider in the first JitD Quest, its trapped in that room.

There is a entry in the FAQ that addresses and forbids the OL from playing Crushing Block in such a way as to make a level unpassable:

Q: Which map items count as obstacles for the Crushing Block trap card?
A: Crushing Block may never be played in a space adjacent to any token (or built-in map element) that blocks movement. The reason for this is to prevent the Overlord from sealing a hallway completely and preventing the heroes from ever progressing. This is a list of all relevant obstacles, current through the Road to Legend: Boulder, Crushing Wall, Rubble, Water. (Villagers are figures, not map elements.)

PetriH3 said:

Edit: Remy beat me to it. Moral of the story is you can be mean with your traps, but you can't shut down a level.

Yes to the first, no to the second.

If you drop a crushing block in a narrow corridor (2 spaces wide) and this traps your monsters as a result, that is too bad and your large monster is stuck.

In terms of making a quest unsolvable, the crushing block trap specifically prevents (and the faq has clarified) you from placing a crushing block trap adjacent to any other movement blocking obstacle like other rubble or water, for example. The idea behind this is expressly to prevent any area from becoming unpassable by the heroes. Anything else, like a pit or an ice patch, lava, mud, etc. can technically be moved through (though sometimes with damage being involved...but since it doesn't block movement of everyone lacking the acrobat/fly abilities you can go right ahead. and play those traps there. A fun thing is to play a block next to something that deals damage and put your hero into that space. Drop a block by a pit and do an extra damage. Drop it by lava or right after the hero just came out of those scything blades and put them right back into it. You can't make the dungeon impossible/impassable, but you sure can be nasty.