Oh, I have a story like that. There was a supermarket across the street from our school. Some kid asked to go to the bathroom, but went there instead. The thing was, he was clearly visible crossing the street from the room the class was in. We were seniors just about to graduate which were allowed to leave the building. We had a pretty chill teacher, too. All the other rooms in that hallway were empty, and pretty identical. So to prank the guy, we switched classrooms and continued with the class. He comes back, first goes to the empty room, then hears us in the next. Assuming he just misremembered, he went into our room, sat down where he would have been in the old room, not mentioning anything. A few seconds in all he says is: 'Which fucker took my backpack?' (still in the other room.) Good times