I once slipped on some ice when entering the schoolyard, completely went down. Perfect timing.
Another time in school, I was absentmindedly chewing bottom end of my pen, and it burst so I had ink over my mouth. Then, I tried to hide it with my hand, but the teacher noticed I was being weird and asked me to move my hand, so the rest of the class got to see.

Moral of the story: Don't chew your pen! It's a bad habit, and will lead to public humiliation in front of people who you already hate!
There will be more, but these are the ones that first come to mind.