Well, I'll add a story of a simple light fixture change that sticks in my mind. I was putting up a hanging pendant with a metal frame, 3-core flexible cord. The ceiling outlet was the loop-in type, so live to the switch fed through one set of terminals.

I'd verified the wall switch was in the hot side of the line, the panel was the far end of the house, it was getting late in the day anyway, so the extra light from the adjacent hallway was handy, so I didn't turn off the lighting circuit. Hey, I'm not touching the live loop-thru terminal, why bother?

I had the pendant frame kind of wedged on my arm as I made the connections, starting with the earth. Somehow I got distracted for a second, turned back to the terminal strip and managed to stab the earth wire from the flex straight into the live loop-thru terminal. Ouch! [Linked Image]