Her tail, and her girly parts. You never know when she is going to need the tail handled and/or her girly parts washed.
I used to have a mare who was like almost perfect, until you put a wash cloth near her parts. It's the only time I ever saw her kick anything other than the jackass. She got my vet square in the belly because he wasn't expecting it either. She was such an angel otherwise.
"Light travels faster than sound. That's why some people appear bright until you hear them speak."
Larry the Cable Guy