A mechanic at our co-op almost had her thumb severed while teaching a class on bike maintenance. ironically, she was demonstrating the dangers of incorrect chain-lubing technique on a fixed gear, got distracted by a question, and her hand was sucked into the fixed cog. Afterwards she stopped teaching and has been a bit ***-shy about working on bikes.

personally, I've had a lot of small injuries while working on bikes. they've all served to help me better understand what not to do and to think through what could potentially happen if the tool i'm using slips under force. and i'm extra cautious when working with someone else, to make sure they don't slip and hurt me.