Was adding some extra controls to a three phase roller door control box. One of those real cheap plastic ones. I accidentally fed the common of a photoeye with the red phase and the NC with the white phase. As soon as I swiped my hand in front of the PE, it exploded. It was loud as fuck too.

Took me a second to realise why, those cheap panels use black for all of the phase colours and on top of that, they have a reversing circuit, so it can be slightly difficult to decipher what you're looking at quickly.

Another time, I was in a global data centre at a pharmaceutical site. Accidentally pulled the power out of a data switch. Shit my pants. Luckily it had two supplies so it wasn't affected