Today I learned I've been running my desktop, monitor, the Internet gateway, and a horde of small devices off an outlet that was wired wrong: hot and neutral were reversed. It's one of the only outlets that was wired for 3-prong sockets when we bought the house.
I discovered this because I bit on the inexpensive CyberPower UPS that was offered as a deal on the front page this week, and plugged it in for the first time this morning. I immediately noticed that the "Wiring Fault Indicator" light came on - useful, that. I have a basic outlet tester, plugged it in, and lo and behold the red light comes on there, too.
I've wired a dozen or so outlets so far in my life. It's not hard! How the hell do you screw that up, or at the least, how the hell do you not test your work afterward to make sure you've done it right?
I can top that. Many years ago, at a summer job I had while still in school, the owner of the computer store I was working at got his buddy to wire a new showroom. The new setup included strips of outlets permanently mounted on the walls, just above floor level.
We discovered that there was an issue with the wiring in spectacular fashion. One of the systems was a fairly high-powered (for that day) setup from a company called Alpha Micro; basically a PDP-11 clone, but using the then-common S-100 bus. It had an external hard drive (the old school top loading removable media "washing machine" style, as this was circa 1979 or 1980), and multiple CRT terminals around the store connected via RS-232 wiring run through the walls.
When we plugged everything in, there were fireworks. Sparks coming out of equipment, crackling sounds, and the aroma of burnt electronics everywhere. Lots of magic smoke was let out that day. The path of destruction was truly epic -- disk controller and RS-232 ports in the system, the external hard drive, and all of the CRT terminals fried. Thousands of dollars in damage.
A post-mortem revealed that the idiot buddy had neglected to wire the protective ground on the problematic strip, and had gotten the hot wire pinched in a joint (so the metal enclosure and protective ground prongs were all hot). The unfortunate victim system was located in a corner of the room, and as luck would have it the system unit was plugged into one wall (hot ground), and the external hard drive plugged into the other (good ground).
Fortunately nobody was injured.
Some friends bought a late-1960s house which had aluminum wiring (fortunately, one of the later alloys that didn't corrode so aggressively). The previous owner was a double-divorcee who decided that instead of screwing up a third marriage by pretending he knew what he was doing, he would screw up a house instead. Did a lot of work, superficially it all looked nice, and then my friends started doing their own renovations. The first thing they found when cleaning up the laundry room was that this guy had wired copper to aluminum using standard wirenuts, instead of the required self-sealing Noalox nuts. The second thing they found was that he had also done this in the attic for a can-light project, and left his Al-Cu feeder ties lying in the cellulose instead of routing the NM cable through a proper J-box. After that, they removed a partition wall and found even more un-boxed terminations, which were tucked back inside the wall with cabinets installed in front.
A few years ago when I went to replace a bathroom light fixture I discovered that it was powered via a piece of flexible armored cable with the wires poking out (no box). Fixture was just screwed into the drywall, with a big ragged hole behind the fixture. Turned what should've been a half hour project into a major PITA (install a box, repair the drywall, then mount the new fixture properly).
I'll bet the fixtures in the other two bathrooms were done the same way (haven't checked yet).