My workshop has a low ceiling, with pipes and shelves in odd places, and doorframes at just the wrong height. That's the usual reason. I once managed to catch a piece of wood I was turning round to sand to other end of on a pipe, it bounced off and smacked me in the temple, which caused me to drop it on my big toe, causing me to hop and hit my head on the same pipe.
Other causes were:
a narrowboat I wasn't expecting to be there in the dark
forgetting the hardtop was on my car
calling a bouncer a -ing -tard who's brain had come in a box marked *batteries not incuded but no one noticed. that one was my fault, to be honest.
Biting the hand that feeds IT
