It's amazing what you find in buildings old enough to have seen all generations of electrical standards.

The oldest part of my sister-in-law's place dates to the 1790's (I know: in America 100 years is a long time; in England 100 miles is a long way) and it has receptacles that are a never-standardized bastard union of NEMA 1-15 and 2-15, which means you can't be quite certain if you've got an ungrounded hot and a neutral for 120v or two hots for 240v. God only knows why they were ever manufactured. I've never looked at their panel, but I assume it's a horror show.

I've seen live cloth-insulated wire in older homes I've lived in, and found a mystery fuse box in a triple-decker in Boston. As far as I know, I've never lived anywhere with live knob and tube, but I've definitely had it abandoned-in-place.

My all-time favorite was the house I had with a five-gang box populated with switches. I don't need to add that the multi-location circuit wired through that box was wired wrong.