In 15 years, I've only been screwed over twice.
One time was a bad check I wasn't able to collect on but since the woman also go evicted from her house, I guess I wasn't a priority to pay.
The other time was a guy with a early 90s Bonneville. He called and said he needed it detailed to sell and I gave him a price for wash, clay, polish, wax and full interior cleaning and protection. I specifically told him any engine detailing would be extra and he'd need to sign a liability release. Anyway, I get to his house and only his kids are there, plus a check for the price of the detail (didn't include the engine upcharge and he wasn't there to sign off on it). The car was pretty dirty, had several old red stains in the back that wouldn't come out, but otherwise the car turned out really good. His wife comes home before I leave, and we go over the car together. I show her the red stains and explained to her the carpet would need to be redyed or replaced. She said she understood and that the car looked great and she was sure her husband would be thrilled.
About 2-3 hours later, I get a call from the husband, only he is trying to act like a new customer. Not sure why he thought I wouldn't recognize his voice. He asks what is included in a detail and I tell him. He then asks about the engine and I said I only do engines by specific request, the cost is extra and due to additional liability, he'd need to sign a liability release form. He then told me who he was (duh) and said he wasn't happy with the car because I didn't clean the engine. I told him again I didn't charge for it and he wasn't there to sign the form. He then mentioned the red stains and I explained to him that old red stains rarely come up and he'd need to redye or replace the carpet. He then griped about the slots between the spokes in the wheels (those three spoke wheels with the 3 slots in each spoke) and I told him the coating on the wheels had deteriorated in those slots and the wheels need refinishing. He then said something like "you aren't much of a detailer" and I replied "you must mean restorer because that's what your wheels need".
He told me he was sending his wife out to pick up the check she'd given me (60 mile round trip!) because she shouldn't have given it to me in the first place. Told me I could have it back once I cleaned the engine and the wheels to his satisfaction. His wife shows up a couple hours later (by this time it was nearly 10 PM) and she was PISSED but at her husband, no me. She felt he was being completely unreasonable and that the car looked great. I handed her the check and told her I was sorry she had to drive out but she said I didn't have anything to apologize for.
Her husband calls the next day demanding to know when I'd be out that day to finish the car. I told him to f-off because there was no way I was going to do anything else to that car and him still find an excuse not to pay me. He is the only customer I've ever cussed out and I guess since I never got payed, he wasn't really a customer, just an obnoxious a-hole.