The freeking freaking size.
Once I was played football and my friend jumped at me to tackle me. I ended up getting his skull directly into my nose at full force. Very painful.
Having to use glasses while doing everything is pretty painful. I once got hit by a soccer ball / football at full speed straight in the face. The glasses broke apart and cut me a bit.
Which is why I remove them before doing sports. (Although I can't see stuff from far.)