LOL - I wore my youngest in a sling for many, many hours a day and we had lots of accidents! The most common was when he was a tiny babe and I would always bonk his head against a drawer while I was putting away laundry.
The worst, though, was when he was about 18 months old. We were at the post office and I must not have had a mei tai with me because I had him on my back in a ring sling. As I was leaving, he leaned way over to look at something and so when I walked through the door, his face got smacked hard on the door frame. Oh, I felt so bad! I think I cried as hard as he did. We sat in the car in the post office parking lot nursing for half an hour until he felt better. Poor thing!
But of course it's no worse than what could happen if you didn't sling. I had my eldest in his infant carseat (this was before I had ever even HEARD of babywearing) in the video store one day. I set the seat down to read the description of a video and promptly dropped the cassette smack on his face. I guess these little things just happen, no matter what you do!