So I saved our contractor hours of aggravation today. (and myself probably hundreds of dollars of money paying them for the time to figure out a problem). He was installing a light fixture in the upstairs hall next to my office. When he turned the breaker back on after he'd finished nothing happened. His assistant was downstairs so contractor proceeded to flip all the breakers off and on in the breaker panel in the master bedroom upstairs on the other side of the hall from my office while contractor watched the hall lights to see if he was trying the wrong breaker. Still no power. Then his assistant commented that the downstairs bathroom was also dark. I was only half paying attention, mainly after my office went dark and the internet stopped working because the internet modem was now rebooting from the breaker affecting it getting turned off and on. I asked what was up and when he explained I said "Oh, it's probably the GFCI in the downstairs bathroom." Contractor was like "um, ok, we can try that..." and called down to his assistant to check it. That fixed it. Contractor sheepishly admitted that he would've never thought of that because current code is that GFCI outlets are supposed to be the only thing on an individual circuit. (you can have multiple bathrooms on one, but you can't include other lights on them so that you don't have a GFCI trip leaving the room dark.)
Thankfully we didn't need a permit for this project since no electric was being changed. If we had he'd now have to recircuit the downstairs bathroom GFCI onto the compliant GFCI in the upstairs hall bath, which got corrected when he did the reno of that room a year ago. A change like this could easily add several $1000 or more to this job because more walls would need to be broken open and then repaired, repainted, etc. That change for the upstairs bathroom last year was easy because all the walls were already opened up so it was just a minor step in a big project.
It's shear luck that I knew this quirk about the electric in our house. About a month ago the light switch in the downstairs bathroom broke so I had to replace it. To kill the power to it I tried hitting the GFCI switch on the outlet rather than have to figure out which circuit on the panel would kill the electricity to the switch and also since it was right there, not all the way upstairs in the bedroom. That had worked, making the repair of changing out the broken switch easy. I hadn't known that the circuit also included the upstairs hall light but it made sense since today the downstairs bathroom was also dead.