LLB's post in the Wednesday thread made me remember mine, but it likely pales in comparison to his:
Dislocated my jaw playing fives a few years ago. They decide to pop it back into place and the lovely nurse asks me if I wanted any pain medication before hand. Now, like Xylopwn, my shoulder regular pops in and out, and to get it back in can hurt a lot, but it's never been that bad, not since the first time.
But good god. It felt like she'd drove a bus into the side of my face. I was delirious afterwards for about 30 minutes unable to string a coherent thought together.