I have done some awful, awful things in my youth.
From childish (I was a teenager) - asking the girl who snogged my boyfriend and thought I didn't know about it to pass on a letter to the 'popular' guy in our year at school, unknown to her I had actually written it from her and declared her undying love to him, in rhyme.
To funny - a guy my friends and I used to go out clubbing with always used to be an arse after a few drinks, we all used to go back to his house after the club and he always ended up falling asleep, he was very, very house proud and made us take our shoes off etc (all perfectly reasonable with the benefit of hindsight) and we found it quite annoying.
One of the nights we were there after he fell asleep we went to the 24 hour garage and bought all the tin foil they had (6 drunk people going in and buying about 30 rolls of tinfoil raised some eyebrows) we then went back to my friends house and covered everything in tinfoil, everything from the fruitbowl and individual bits of fruit to the couch and rug and pictures on the wall, it took us about 2 hours.
It was hilarious (and we were forgiven after a few weeks).
To downright horrible - A particular ex of mine (he was really nasty when we were together) used to come into the nightclub I worked in (the only nightclub for miles around) although he never did anything when I was at work the sight of his face pissed me off so much that I lied to the security staff and he got banned, and because of the 'pubwatch' scheme he was banned from all the other pubs too. I killed his social life in one swoop.