Nowhere near the worst things I've done but still probably the best to share.
1. I was at some friend of a friend's party, was quite a shit party, pretty much a meeting of wronguns on the weekend after dole day, the home owner was playing music through an old TV, one of the ones that had a DVD part on the side. After arriving I pretty quickly decided this wasn't what I wanted to be doing for the weekend even if the other options were limited so I bailed but before I went I thought it would be funny to put into the telly a Justin Bieber album, the owner wasn't in the house at that point and I turned it up full blast and left to go to the local clubs.
Next day I wake up and find out the guy already had a noise complaint sort of thing so the police had came round and confiscated his TV as that's where the music was coming from, had to wait about a month til he got it back...
2. Years ago after watching the Prison Break finale I went onto Twitter and found people that had posted "#S4E13" or whatever the second last episode was and searched chronologically and posted the ending to all the people that had recently shared how much they were looking forward to the ending.