My friend has a hellll good one.
At my school there is this 3 story building with an awning at the very top. A peice of string hung down, well actually it was lots of bits tied together, and it came down half way up the wall of the second story.
We decided that we wanted to see what was on the other end of the string. So we decided to make a human pyramid to get up high enough so my friend could grab it and pull it down. So now we have this pyramid, its about 4 people tall (standing), and my friend is on top reaching up while the people below him hold his ankles so he doesnt fall.
Hes about 3 inches away from the string, and he shouts "I can reach it if I jump!". So he jumps. But the people below him are still holding his ankles, so he just hinges forward so hes upside down, and the pyramid falls down.
He smacked the shit out of his head, and went to hospital. We never saw him again.
Jokes, yeah he came back to school weeks latter, he just fractured his skull.
Oh, and we found out what was on the end of the string (second time's that charm), it was an old shoe.