I just bought a house built in the late 90's. While upgrading some lights in the bathroom, I replaced a recessed can light with an LED version that uses an adapter for the bulb socket and simply fits over the can. The only wiring involved is a ground wire.
After getting the new light setup, I tested it and it worked just fine, including when I had the ground wire hooked to the can. As I was pushing the cover onto the can, all the bathroom lights went out. This included the shower & toilet cubby next to it. I also found that the other 1.5 bathrooms also didn't have working lights anymore.
Fortunately the wall sockets still worked.
I find this strange, but evidently code allows bathroom lights to be all on the same circuit as long as the outlets are on a different circuit. I think I still want to separate them out, but that's a different topic for later, I think.
I went to the breaker panel and tripped what I thought was the correct breakers, but I still didn't have lights. Then I flipped all the breakers to make sure I got the right one and still nothing. I ended up getting out my multimeter and taking the front panel off the box, then checking that the breakers were working correctly. They were, but still no lights.
I've searched the whole house multiple times and there isn't any subpanel or anything else I can find with breakers in it, except for the dedicated breaker for the AC unit outside.
I'm going to self-answer, since I finally figured it out accidentally. I'm writing this up so that other people might benefit from my weird solution.