A classic army practical joke:
Switching boots. You need a few people who have different shoe sizes and then you start switching boots with somebody who doesn't know what's happening - you might even try to convince him his feet are growing again (on daily bases). My brother and his friends were able to make three switches before the poor sap realized.
(personally I was never in the army
- they introduced proffesional army and stoped with conscription just two monts before I'd have to go - aren't I a lucky basterd :bleh: ).