Three, one intentional, one accidental, and an emotional one.
First, when I was in year eleven, my friend was being the dick he usually is, so I gently nudge him to the side as we go past a low wall, so that he missed his step and the wall (about shin high) made him fall forwards, smashing into the floor, no lasting injury, but it was hilarious, and he got quite mad.
Accidentally, I threw a rock, but was too young to know exactly how throwing things worked, and it stuck in my hand too long, (despite letting go) and did a perfect slingshot round flying out behind me, and into my mothers arm, which was difficult to explain.
Emotionally, I almost drove my ex to suicide once, when I was talking about someone I'd slept with without protection (who I trusted implicitly) Luckily there was someone around to help out, considering I was unaware, and three thousand miles away.
(There had been no warning of this before hand, I'd helped them out before, and was fairly confident I couldn't drive them to such an act, we are still friends, going to go see them on tuesday ^_^ )