Friend of mine once was bit by her horse in shoulder when she was brushing her front legs. She was crying. Not because she was hurt, but because she NEVER expect her horse do it (and she indeed loves her and takes good care of her).
I agree with DGW, we just have to be smart enough to understand what is coming and prevent that. I've kinda seen I'll can be kicked but still was insistent (I believe in presence of other person, otherwise I'd just let her go). However there are smarter and sillier horses (same with dogs, cats, etc.) :)