I can't think of anything that I've done tat's really bad....I've forgot about the other end of the barbell, and it came crashing down to the ground when I took the plates off one end...I'm sure lots of people have done that....right?
But I saw one guy, he was going to do hyper extensions, and for some reason the thing was placed in under the cable crossover machine and he just left it there. Anyway, he didn't check to make sure the peg was in place that holds the foot pad on, so he got up, hooked his legs under, and dropped his upper body down to start the exercise, and the pin wasn't in, he came flying over the thing, did a flip and scraped his head all down the cable machine.
It was pretty bad. Even if he'd moved the stupid thing where it should have been, he wouldn't have hurt himself so badly. anyway, I learned to always check the pins