33 years training, there are a few standouts I'll never forget.
The worst I've ever witnessed in person are:
1. 1989, middle of summer, 90 degrees in the gym, no air, everyone sweating like crazy. Dude was doing EZ lying extensions with a thumb less grip, 90lbs on each side. The bar slipped out of his hands and lands on his mouth, breaking every front tooth. Spotter could not react fast enough. When they took the bar out of his mouth his entire jaw was basically hanging off, blood everywhere, teeth on the floor. Very disturbing scene.
2. 1995, a young fat guy was close grip benching 315 in a ballistic fashion. The old thumb-less grip strikes again. Right after I said to my training partner "he's gonna get hurt" the bar slipped out of his hands (at full lockout) and lands on his upper chest, he starts screaming, about 5 people run over to rescue him. I heard he broke both clavicles. Luckily the bar did not land on his throat. Never saw him again after that.
3. Powerlifting meet circa 1988. Heavyweight class lifter attempting an 805lb squat. Takes it off the rack and walks it out. As he was getting set I could tell something was wrong. As he descended he yells out in pain as his right knee starts bending inwards. He fell to the floor and weight barely missed landing on him. The guy was screaming in agony. He tore all the major knee ligaments and most quad insertions on his right leg.
And no, I did not enjoy seeing any of these.