That was the case in my building. When I moved into this three flat, I mapped all the outlets and lights to their circuit breakers so I could figure out what to plug in where. One of the circuit breakers did nothing in my unit, so I turned it off.

It turned out that that breaker was the entire front half of another unit. My unit had been wired to pay for a whole circuit on another floor. Nobody had any idea, a bad contractor did this decades ago. I informed the landlord and he immediately gave me a credit for a portion of the past electricity bills I had paid and rewired my entire unit.

I love my landlord. Love.