Who pays for the misdiagnosis?

A couple cars that I serviced the other day got me thinking.

One was a 2011 Mercedes ML350 that came to my shop with brake lights that flashed in the wrong spot and set a "brake light failure" notice on the instrument cluster. We diagnosed a bad rear SAM module. It cost about $400 just for the part.

However, it turned out that we were wrong with our diagnosis. It didn't need a $400 module, but rather a $16.50 wiring harness.

I told the customer that I misdiagnosed it and he owed me nothing for the computer I put into the vehicle and he was happy.

Another car was a 2002 Honda Accord that would not go into reverse. The tranny shop sent it to us after rebuilding the whole transmission. Why? The vehicle still didn't go into reverse and they found a TSB that said a reflash would correct this.

In this situation, the customer would pay for a whole transmission when he only needed a reflash. However, what we found out was that the software was up to date and that there was a defect with the rebuilding of the transmission. They took the transmission back apart and corrected the problem.

What I wonder is whether most shops are like my shop or like the other one? How many of us tell ourselves "it needed it anyway" or "it was going to need it?"

All I know is that in the cut-throat business of auto repair, where customers do not like paying for diagnostics and shops throw parts at cars, it almost seems natural that these diagnostic mistakes are going to happen. I'd even go so far as to say that the customers who don't treat technicians like professionals would deserve it, because even doctors don't pay for a patient's misdiagnosis.

What's your take?