Here's on I did a few years ago.
Location: Highschool.
I was sharing a locker with a friend at the time. I went to his locker and unscrewed the joints during class, then put the door back in place and made it look normal. (Sort of easy, since the door always sticks anyways. It was dented from being kicked closed and you had to really pull on it to open it

)
Lunch comes around, i make casual chit-chat with the guy next to his locker. friend comes and undoes the lock and gives the door a strong pull.
It nearly flies out of his hand and across to the other side of the hall.
He looks at it, yells "what the fuck?!", sees me doubled over with laughter and chases me down the hallway waving a green 6 foot long locker door above his head.
We got stopped by a teacher that said it was vandalism and sent us to walk to the office (attracting many stares along the way and getting stopped by lunch monitors that asked us why we had a door).
We handed in the door and it's screws (which took FIVE DAYS for them to get around to putting back on, including a weekend), and the principal told us he'd have us both clean up around the school sometime. We could hear him laughing when we walked out. XD