Once upon a time, when I was a teenager, I was in a cellar. A family members house.
I was changing a ceiling lamp holder, the white plastic ones. It was broken and faulty.
The switch was up, to denote no electricity to the lamp holder. It was a one way switch, one switch, one light.
As I was disconnecting the lamp holder, I got a full blown electric shock, that made me fall on my back. Biggest electric shock I have ever had.
Turned out the switch on the wall was upside down, one of those old fashioned round ones. What I thought was off, was actually on.
Big lesson learnt, whenever doing electrical work, switch off at the mains or take fuse out.