When playing football once, a bad challenge made me land on my hand and I heard a crack. I thought (and so did the physio) that I had dislocated my finger, so he, (and one of the subs) took it in turns to try and yank it back into place.
After having no success, and then going to hospital the next day, it turns out that I had broken it in 4 places. No doubt before the physio got hold of it, it was only broken in one!