There are always two sides to a story. this will be one from the Other side:

A guy brings in an MG with a SBC in with an o/heating problem, engine compartment covered with dried rusty coolant. He claims another local shop piped him and said it was fixed. He says it overheated within 3 miles of the shop and that after getting piped he wasn't going back. So he had it flat bedded home and then to us.
OK. A quick check revealed the fans weren't operating and there were really bad crimps on the wiring. In fact the whole car was a rats nest with a mix of mismatched splices connectors etc. I recommend a complete rewire @ ~5K. he asks can we just fix the fans. I said yes with a disclaimer of liability on any other electrical issues. About 700.00 total with the time already consumed. OK.
So we do a complete rewire of the fan circuit, relay, probe etc. I call him and say it's done. he requests that we put at least 30 miles on it to verify it's no longer O/heating. OK. Call him and say all is well. He says go ahead and clean the engine bay and polish the stained aluminum etc. as he is taking it to a show. We proceed to do so and a quick detail. I email him a 1200.00 invoice. He shows up with his wife throws a tantrum and says 1200.00 to fix a couple of connections is outrageous and stomps out and gets in the car. I actually apologized to his wife for her husbands outrage and said a lady should not have to be subjected to that. She goes out talks to him. Comes back in announces she is a lawyer and they are willing to pay 500.00 IF I sign a pre prepared statement releasing them from any further claims. PO'd yes, worth the time or money to pursue NO. Sign the paper take the check. 2 days later the car is on line for sale at a local classic car dealer. I contact a person who knows the dealership owner quite well. He tells the owner to pick the car up as their inspection revealed electrical issues he does not want to be liable for. Don't know where it went after that.
Fortunately I have only had a few similar incidents in 25 years of business. And with each one I became a little smarter but there are a lot of "slick willies" out there wanting it for nothing.
One of the things I learned not yet mentioned, "The quicker they want it done, the more likely they will be a problem when it was time to pay." BTW we had about 16 hours in the car.