“Bag under the door” trick :Close the open end of the bag (press down on it with something straight, like a long ruler) and the person on the other side the door gets to have the bag burst open all over them.
If you are in a dorm of practical jokers, you’d know to keep a ruler handy.
This happened to me in our dorm room my freshman year. Only it wasn’t a wooden door – we had a metal door in a metal frame. When you “penny” someone in that way, it’s serious. With wooden doors or jambs there’s some “give”, but metal doors and frames are solid.
I’d forgotten about the shaving cream trick, thanks. I witnessed it but never had it done to me. Another one was to cover the door opening (from the outside, of course) with newspaper. When the occupant would open the door to come out, they’d see wall of paper and kick or punch it out. What they didn’t see was the metal cabinet that had been placed in front of the door right up against the paper. No serious injuries and word got around pretty quick so people stopped falling for it.