We have started a house "lab notebook" documenting stuff as we find it so that we aren't starting absolutely every project from zero knowledge.

We replaced the bedroom ceiling fan this summer, and discovered some "interesting" things about the rest of the wiring on the second floor. I added some notes for future me so that when I go to replace the sconces later this year, I don't have to figure out the weird wiring for them a second time.

I highly recommend doing this for your own sanity and for that of any future owners.