I watched a wood machinist cutting his finger nails on his circular saw once. (He got away with it.)
I watched a manager in my department absent-mindedly poking a finger through the grille over a large cooling fan at the back of a rack of electrical equipment once. (He
didn't get away with it - he yelped in pain as the fan took his fingernail off!)
I was trying to take the pedals off my bike to pack it away for a cycling holiday. I remember thinking "Colin - be very careful taking the RH pedal off - if the pedal spanner slips, you could gash your hand on the big chainring." I went ahead, the spanner slipped and I gashed my hand on the big chainring!
Once, after a few drinks with mates, I decided to show off by getting out of a car while it was still doing 20 mph. I sobered up in about 1 millisecond as my feet were whipped away from under me and I was dragged along clinging onto the car door handle. I lost a shoe and a lot of skin off my toes!
In my local gym I'd got bored of sprinting on a fixed-wheel exercise bike with a 20 kg flywheel so I decided to stop pedalling and dismount...
A tall friend of mine decided to cycle home from a party rather the worse for wear. He chose to use the local canal towpath, headbutted a bridge and ended up in the canal!
Oh, soldering irons - once, I realised that mine was on so I made sure that I
didn't pick it up by the handle! (I melted grooves in my thumb and forefinger.)
I was interested in testing the principle of electromagnetic induction so I connected a large choke coil from an old TV to a 9-volt battery while holding the ends of the wires. (When I broke the connection a multi-kV shock threw me clear across the room!)