@Dtl reminds me of a conversation I had with a colleague once:
Them: "have you worked on these before?"
Me: "yes, a few times..."
Them: "I've got the lid off, any advice?"
Me:"Um..."
Them:"arghhhhh!"
Me:"the heat sinks are live, not grounded?"
Them:"thanks."
@LewisWorkshop @Dtl shocked myself on a network appliance a while back due to this. they had a couple of TO220 packaged diodes bonded to a heatsink, and the tabs were at mains potential. gave me a dead arm for about an hour.