The worst electrical accident I've had was discharging a 30 kV EHT system through myself. One minute I was aligning a tube in a projection TV, the next minute I was waking up, upside down on the other side of the room with my boss and two other repair technicians asking me if I was okay. I distinctly remember the smell of burning flesh, and the fact that the palpitations didn't subside for two days.
The worst mech-eng accident I've had would be when I forgot to remove the mole grips that were clamped on the clutch basket of the motorcycle engine I'd just finished a top/bottom rebuild on. I went to thumb the starter to turn it over a couple times slowly, just to move some oil around, and unfortunately the acetone I'd used to clean off the gasket material had evaporated enough inside the engine to fire the thing up (the fuel system wasn't connected, so I didn't expect it to run.)
The engine fired up and as the timing wasn't done yet, it immediately shot to about 11k rpm for a tenth of a second or so; I know this because my datalogger was attached. I immediately lost interest because the mole grips came off the clutch basket and hit me in the back of the head and knocked me flat out. Fortunately it was a glancing impact because if it hadn't been it probably would have fractured my skull and/or killed me.
Since then I've always been very, very careful about what I leave attached to rotating machinery!