A number of jobs ago, I used to do on-site service for small companies. One place I went to was a furniture store, and we did their POS systems. I was called out to fix one of them, and they hit me with the old standby of 'while you are here, can you fix this other one?" I figured, what the hell, I'm getting paid anyway, and it was on covered equipment, so I poke at it, and got it working in a couple minutes.
Now, we were not allowed to accept tips, but this time, I was forced to accept it as a good will thing. $20 for plugging a cable into the correct port and fixing a POS that was down for some time prior to that? I'm good!