This one is a classic from my GM times with Paranoia. When the troubleshooters near a corridor that their security clearance just allows, and arrive about at the middle. Then suddenly some scrubots come and wash the wall, floor and ceiling, revealing a much higher clearance color, and of course some security cameras. Now watch them run as the Computer starts signing up list for termination, and even if they flee, they still have to face two scrubots coming from both directions.

My favorite is to make red (walls splattered with blood) or green (slime) wall, and the to clean them up to indigo. Once I even made the corridor white, but then all my clones started in unison "where did the corridor vanish? I can´t see it."