It isn't just plumbers.
One of my customers told me about a carpenter who doubled his quoted price because "the job turned out really well."
A refrigeration mechanic on a maintenance contract told a customer that her air conditioner was about to fail and she needed a new one. It even quit working right after he left. She was suspicious and had it checked and another refrigeration mechanic and an electrician. We turned the switch back on.
My ex took her new car back to the dealer because it occasionally stalled on the highway. They told her that it was supposed to do that. A different dealer replaced the fuel pump.
Electricians do it too. Dishonesty isn't limited to certain trades.
Maybe we need a feedback system like ebay.
