While I was at Caltech in the 1950s, wicked upperclassmen entered a freshman's room, coated the center terminals of all the light bulbs with clear Glyptal, and replaced them. Of course, none of them worked, and the freshman could not see anything wrong with them. Naturally, he suspected that a fuse had blown, so he went to the fuse box at the end of the hall and examined the fuse. There was nothing wrong with it. At that point one of the wicked upperclassmen happened by & asked what he was doing. The frosh explained, and the upperclassman looked at the fuse and said, Someone has switched fuses on you. This is a size 15 A fuse, which is too short to reach the bottom of the socket. You need a 15 B. (Actually, the A stands for amperes.) I forget whether the victim actually went to a hardware store & asked for a 15 B fuse.