I once did pushpresses, and for the warmups I got a little nonchalant, and I pushed the bar straight into my jaw resulting in visual damage to a tooth. I still have this, it is quite visible, and it is one of my upper front teeth. I wear it with pride, doesn’t bother me.
I was trying a new PR in the front squat in the school gym, bar was loaded and I went down, on the way down, I felt that something was wrong, and I hurried and got the bar away from me, but still I could not control what happened next, I fell backwards, first slamming my butt to the floor, then slamming the back of my head to the floor. For a brief moment I was unable to move my body, while still being conscious sort of.
When I recovered from this, I tought it was awesome. Really hardcore. Everyone else at the school gym seemed really bothered, and they came running to see if I was okay. I was a bit angry, and I was sure I would have made the lift if there were other serious lifters around me, and not all those disco trainees.
Another time I was doing a squat workout, and when I was pushing with all force in the last rep of one set, I did shit my pants. I went to the bathroom, to clean it up, and noticed that there was a distinct stain at the center of my behind. Never mind I had to complete the workout, so I went back in.
Yet another time I was doing heavy from squats for fives, and when I ascended on the third rep, blood started coming from my nose, I just kept at it because I had two more reps to complete. After that I went to the bathroom to clean it up, and I looked like someone had punched me in the face. Blood everywhere. I stuffed some paper in my nose and continued the workout.
The worst case was when my most serious injury happened. I was squatting, and suddenly my lower back snapped. It was so painful that I took a cab to the hospital. That is the only time ever I have aborted a workout I have started. It took me more than a year to recover from that, but all is well now.
Some other fun accidents I have heard about or experienced:
At a gym where I used to be before, a dude was doing some heavy leg presses. When he was finished with his last rep (I was spotting him), he slammed the weights down on the supports, and then the entire set of weight and the bar holding them on the right side broke and slammed into the floor with a loud bang. The look on his face was priceless. A closer inspection revealed that this was something that was due to happen. I was only happy that I stood on the opposite side.
Then you have the classical incidents were people do lat pulldowns, and the wire snaps, and the bar comes crashing right into the forehead. Happened twice at my old gym.
Another bloke I know was doing seated smith overhead presses. When he was finished, he secured the bar overhead (he tought). It was not properly secured, and it came crashing down right into the middle of his head. He completed the workout, but not without pain.
Another bloke at the same gym was doing jumps of a box. He jumped off the box, and then accelerated right into the roof with the head first. That was enough for him that day.