I was riding a hybrid bike on dirt trails and hit a bump and the stem kind of collapsed and the front wheel goes off track suddenly. If I'd been going downhill it might have been pretty spectacular but it was just a sort of graceless fall.

Snugged it up enough with my multi-tool to finish the ride but later tightened it up with a liberal coating of Loctite on the bolt. Still have the bike - this was about 15 years ago - and I don't think I have touched that bolt again.