During my BMT, there was this one recruit who was just your typical kind of asshole and the bane of everyone's existence. He was also the superstitious type, and once complained that someone rocked his bed mattress when everyone was asleep at 3am.
So when I booked out I asked my sister to give me a strand of her long hair and brought it back with me. During the platoon cleanup I hid the hair under his pillow.
Amazingly enough he didn't find it (which means he never clean his bed properly) so I went to his bed directly and asked him ""Oi, you got see girl just before you book in ah?""
""No, what talking you??""
I pulled out the strand of hair from under his pillow, and his face turned pale white instantly. He kept quiet the entire day and the day after. I never had such satisfaction with a prank.