About 15 years ago I was the field service engineer for an electrical wholesaler who sold industrial computers. A customer 100 miles away had ordered a UPS for their computer, and it was "coming on, then shutting off." I wanted to talk to their installer, but they demanded that I bring up a replacement and install it, since the original was "Installed by an expert electrician." I explained that if there wasn't a problem with the unit and the problem was installation, they'd need to pay for my mileage, door-to-door and an emergency service call, also door-to-door. This only got threats.
I bundled tools and a spare UPS into my car and drove 2-1/2 hours. The UPS had been plugged into itself by their "expert." I showed this to the customer, wrote him an invoice, and left.