When I worked for somebody else I had a residential no heat call. The furnace wasn't running at all so I checked for power and found none. It turned out the guy had people in painting his basement and turned off the furnace switch when they left thinking it was a light switch. He was really embarrased when I told him. He gave me twenty bucks as I was leaving saying he was sorry I had to come out for something like that.
I still handed in my paperwork so he was charged for a service call.
I wonder if he thought the twenty bucks was all he was going to be out.