In line with the question about Best sysadmin accident, what's the worst accident you've been involved in? Unlike the previous question, I mean "worst" in the sense of most system damage or actual harm to people.
I'll start with mine:
We have two remote wiring closets that are at the end of a 100-foot corridor which has a metal grate for the floor. After we had Cat6 cable installed, the contractors cleaned up all the debris that dropped through the grating to the concrete 3 feet below. A co-worker and I entered the corridor to check on the progress one day but were distracted and didn't notice that a piece of grating had been moved aside. My buddy stepped into air and his chest slammed into the steel crossbar. He was winded and sore enough to take a couple days off, but luckily the steel beam had rounded edges and the size of the opening was such that he didn't smack his head into it or the floor below.
Obviously we learned that areas where the floor is partially removed need to be flagged.