The funniest one features a kid who's trying to blow up a 2-liter bottle. It's full of Coke (I think) and he put something in it (mentos?) to make it foam up, and then he throws the bottle at the ground. It bounces around a bit. The cameraman says, "You missed the top," at which point the bottle's top comes off. The bottle somehow goes bottom end up, and the thrust from the escaping liquid is enough to propel it into the air, and it kind of hovers at about eye level as it makes a beeline for the cameraman.
What I like to do is to click back to the moment of liftoff, because it takes perhaps three-quarters of a second for the bottle to go from ground to THE CAMERAMAN'S FACE, and then keep clicking right after he makes that noise:
Cameraman: You missed the top--NGALK! You missed the top--NGALK! You missed the top--NGALK! (etcetera)
This amuses me.
The other one is a guy trying to jump his mountain bike into a river. You see this very tall ramp on the left side of the frame, and suddenly this guy on his bike comes whizzing into the shot...only when the bike hits the ramp, the ramp comes apart. A piece of it lands in the water.
A lot of fail vids feature nut shots and other non-funny things, and I don't laugh at those things, but the ones where people don't get injured (or not seriously) are usually pretty f-ing funny.