when I was in the service we went thru periods of practical jokes - it was common to arrive in the morning and find someones earmuffs (we worked on a flightline) frozen into a 5kg block of ice. I've also seen some of that 2 part expanding foam poured into the bottom of a locker (result - large foam block containing a pair of boots that then had to be cut free). The best of these however was removing the pins from the locker door hinges - when the owner next opened their locker the door came off in their hand (you can imagine the rest...)
Biggest involved the Base commanding officer - he was a pompous pr*** who used to turn up for his flights wearing a silk scarf...
anyways - he had sharksteeth painted on one of our (his??) aircraft and it looked bloody awful (and frankly, it was embarassing). One night shift, one of the techs painted them out which caused a HUGE stink the next day - a wasted effort as they were just painted back... Recognising the opportunity, the next night the tech moved the afflicted aircraft to the hangar, then painted over the numbers on another aircraft with the numbers from the 'hidden' aircraft and parked it in the same place... next morning the senior noncoms and engineering officers were having a fit until they worked it out...