My sister went on holiday and let me have a house party at her's while she was gone, ended up getting totally wasted despite saying I'd stay sober and look after everyone/the house, before sprinting into the front room, doing a fat handspring over the sofa and bailing mid-air, then when I landed I put my foot through one of my nephew's toys, and completely laid waste to it. The next morning I was cleaning up and found this broken toy stashed behind a sofa, turns out I'd decimated £80's worth of birthday present from the week before. Carefully stacked it up as best I could to resemble what it looked like before, then made sure I was at my sister's when she got back. When she got back a few days later, I 'accidentally' threw a ball at my nephew's head while he was in front of it, he ended up tumbling backwards and sitting on it, my sister still thinks it was him who broke it.
Also hijacked my mate's CV while he was finding out how to print it in college, changed his secondary school to 'Gary Glitter's School of Paedophilia', gave him an A* in 'Fingering Boys', 'Lubing up', and 'Smoking poles', and a U in 'Holding down a job', amongst other things. He printed off 20 copies and handed them in around town, and got loads of shit off his Mum for not getting a job. Funnily enough I still haven't told him either ha.