He just wanted to install a new light in the bathroom, but it didn’t turn out exactly right. It looks like a case of two homeowners in a row who did their own wiring, and believe me, building on someone else’s weirdness only leads to more weirdness. At least this couple hasn’t been electrocuted, which is a real possibility when you get this many wires crossed.
I once had a bathroom/bedroom addition built. It didn’t take long to figure out things were wonky, because when you turned off the bathroom light, the alarm clock by the bed stopped. I could’ve called the contractor back to fix it, but he’d screwed up so many other things, he probably would have just made it worse. I got a master electrician to sort things out -after he stopped laughing. -via Viral Viral Videos
This is why we have properly trained & licenced tradespeople.
I do hate how electrical wiring is done, today. Replace an outlet and several random circuits stop working (if you don't expect it and handle it just right). It would cost just a few more dollars to wire a house without ANY of those elaborate pass-thru / points of failure.