Every electrician has at least a dozen of these. The previous tradesperson who wired a panel with the neutrals and grounds bonded at every subpanel. The junction box behind the drywall - no cover, live connections, insulation touching the bare wires. The 15A wire on a 30A breaker “because the breaker fits.” The three separate ground rods in a backyard connected to nothing. The service panel with the main breaker bypassed by a copper jumper because “it kept tripping.”
I’ll start: walked into an insurance restoration job last year on a house that had burned. Previous homeowner had done their own “electrical work” for about 20 years. Found a 120V outlet on a 240V circuit. Found a whole circuit running on a single 14/2 wire - except one segment in the crawlspace had been spliced to lamp cord. Found a GFCI outlet wired backwards and “protected” by a sign on the wall that said “DO NOT PUSH TEST BUTTON.” Swear on my license this is true.
Post your war stories in the replies. Bonus points for photos (blur any identifying info). We learn from failures - ours and other people’s - and there’s something therapeutic about sharing the worst of what we find.