Freeze Nurse Edna

From Glitches

From the game Maniac Mansion for the NES.

Put one party member outside Edna's room and one outside Ed's room. Have the first person enter Edna's room, then quickly switch to the second person and have him/her enter Ed's room. Ed will do whatever it is he's programmed to do (depending on whether you've made friends with him or not) but when you switch back to the first person, you'll see that Edna has frozen in place and can't move.

Taken with permission from Flying Omelette.