The kitchen has an old Nutone vent hood. The fan didn't spin as fast as it should and the fan blades looked rusty so I decided to replace the motor and blades. I found a replacement motor off Amazon (of all places) and found a metal fan of the exact same design off ebay. I dismantled the vent fan assembly and replaced the motor and fan and tested it and everything worked fine.
A few days later I switched the vent fan on and it did not work. Neither did the hood light. I puzzled over this, wondered if I'd blown a fuse, checked all the fuses - all were intact. I could not figure anything out. I decided that the wiring inside the hood was old and had come apart from my fiddling with it.
A few days later I idly switched the fan on - and it worked! Now I was sure there was loose wiring somewhere.
Then a few days later again I tried the fan, and it did not work. I banged the hood I banged under the fan trying to see if I could jar the loose wiring in place but nothing worked. I left the kitchen disappointed, trying to decide how best to tackle this. I started to think of replacing the entire vent hood.
After a while I came back and switched off the over sink light. There was a sudden rattling sound. I jumped. I was confused for a moment. I was pretty sure I had turned off the over sink light and not hit the disposal button (they are located symmetrically on either side of the sink) but it sure sounded like the disposal. I flipped the switch again. The rattling sound stopped and the sink light came on.
I flipped the switch again. The light went off and the rattling - which I now properly identified as the vent hood fan, which I had messed with and unseated so it was brushing the hood roof (because of all my banging) - started again. It took me a while to stop laughing.
Somebody had gone to great lengths to wire the kitchen light with a single pole double throw switch with the other circuit serving the vent hood. I'm so glad I had accidentally left the vent fan on, otherwise I would have NEVER figure this one out.