When on a level with a jail cell (i.e. The Unusual Suspects), don't place a ghost in the cell to scare them, unless you let the prisoners out (by having the lock, the machine on the side with the red buttons, short out, etc.). If you don't the people will disappear (some sort of glitch), and you will not be able to finish the level (it will say that they are still there, but there aren't).