Well lots of bangs, busted knuckles, cuts etc from this trade! A few come to mind though. Doing control arm bushings in a Grand Prix. The bolt was seized in the damn bushing. I beat and beat and beat on it, finally I got mad and took a swing with my 5lb mini sledge. I guess I moved my back the wrong way, or too fast and put my lower back out. I was crippled for 2 days on the couch before I could move.
Another one was not that long ago, checking u-joints on a truck, had the end in the transmission and just set the back on the rear yoke so I could bend down and get a bolt I dropped. The damn thing slipped off and came down and hit me right on the top of the melon! I swear I nearly saw stars. Funny thing is, is that I had a headache before it hit me, and as soon as that happened the headache was gone! Knocked it right out of me. 
I'll probably think of some more.
2000 Chevy Silverado RCSB LS6 5 speed, LS6 short block, 317 heads, LS6 intake, Lunati 220/224 .575/.575 112LSA cam, Edelbrock Headers, CTS-V accessory drive, Nelson Performance Custom Tune.