True story: When I went to replace my attic fan (motor finally burned out - it had been installed sometime in the 50 years before we bought the house), I found that it had been powered by tapping into the lighting circuit in the hallway (not really an issue for me), running a line to the fan's thermostat box, and then adding a switch leg from there down into the hall closet - so it could be shut off if desired.
However, I found that whoever had installed it left the HOT lead live, and switched the NEUTRAL line. Confused the heck out of me when I first opened it up. I ended up rewiring the whole thing. I fixed that problem when I was replacing the motor.