Wow. Some very important food for thought here.
Misanthropist, your comments about watching out for mundane and routine hazards are really important. Here's hoping that your colleague recovers swiftly, and that you feel better about your actions. Others have already pointed it out, but it's worth bearing in mind that anybody could have loosened the bolt - but fortunately, it was someone who knew how to respond to an emergency situation.
It's always important to be as philosophical as possible about this sort of thing, not least because the alternative can leave you feeling pretty down.
I once knew a guy from out East who, while cutting wood, almost chopped his friend's hand off with an axe. The accident was the product of inattention on both parties' parts. One guy was chopping, and the other setting logs on the block to be split. They developed a routine, became complacent, and failed to recognize that they had become out of sync. In the end, the 'chopee' wound up with some nerve damage, but was able to get past it. The 'chopper', on the other hand (oh god, what an awful pun), had a terrible time dealing with the guilt. I lost track of them, but I imagine that, of the two, the fellow with the axe is probably still the one having the harder time dealing with it. Anyway, I mention this not to draw direct comparisons with the case Misanthropist describes, but to underscore his point about watching out for routines.
All the best,
- Mike