I learned by watching.
I saw a guy pull really hard on a brake spring (drum brakes, automotive) and when it slipped, he jabbed himself in the forehead with the needle nose pliers. He was O.K. after a few stitches.
I watched another guy using a screwdriver improperly and he ended up stabbing himself in the stomach (not requiring a hospital visit) when it slipped.
Yet another guy I worked with was unscrewing the anchor for a very large garage door, and when the last bolt pulled out, the mechanisim (with a few hundred pounds of cable and spring tension) shot up and took out most of his forehead. He almost died and was never the same.
Finally, though I have never really hurt myself working on the motorized bikes, I did do a truly idiotic thing while working on a motorcycle once. I had just completed a loooong total restoration on a '76 BMW R/600 and I was all gung ho to get a ride in. I had to drill the lisence plate so it would fit the European style plate mount. I drilled the first hole holding the plate out in front of me, the hole took forever to drill with nothing backing the plate up, so on the final hole I had to drill, I set the plate on MY THIGH.
As Paul Harvey would say, "You know the rest of the story."
Yes, I went to the hospital. Yes, it hurt.