going off a jump wrong, when in the air your front wheel drops, and you face plant. its happened to much to me that i've attained a great fear for dirtjumping, but still do it.
Wooden Bridges breaking while riding on them. One broke on me last year and i fell really hard on a sharp tree stump cut and bruised leg, and a sprained ankle... went limping home. Couldn't walk for over a week...