first thing that comes to mind is killing my pet budgie

okay: so I didn't kill it with my own bear hands (I let it go out the window, where it couldn't survive the weather conditions) but it was a wonderful pet, and I'd wanted one for ages and my poor brother (who got me it in the first place) wasted all his time getting it as I kept it for about 3 days
other regrets included: punching a nurse and a security guard in hospital (at least with the nurse, she didn't really deserve it, she was just trying to help), destroying my only half deecent peace of school work (which meant I left with no qualifications), pushing a poor baby girl down a hill (when I was meant to be looking after her), and wasted a whole film of camera taking pictures of my naked bottom for a fake facebook profile

oh and breaking my sister's arm

probably others too