I've had my share of dodgy clients, I,m a multi skilled electrician and plumber so work on both regularly. While I was doing some electrical work the client asked me if I wouldn't mind swapping an old radiator in the bathroom for a new chrome towel rail type. She had been very hospitable throughout the job so I was happy to oblige. Turns out a big mistake! It was an old open vented system that had not been drained down for years. I ended up doing a flush on the system and it took ages to fill back up, bleed through and balance out. There were 2 radiators down stairs which would only get slightly warm unless we closed off all the other rads in the house. No matter what we tried we just couldn't get these rads hot.
We gave up close to midnight that day and I explained that it was very strange that these rads weren't getting hot. She was adamant that they worked fine before "we messed". next day we tried all sorts again. She began to get nasty with us and decided at this point she was not going to pay a single penny for any including the electrical. I don't ever like to upset the customer so I bit my lip and proceeded to try and sort the problem. The following day we were back again and to no avail, this was Christmas eve by the way.
That evening her husband returned home from working away. Turns out that those 2 rads had never got hot, only slightly warm for all the years they had been there. That was the last correspondence I had with them, despite numerous phone calls and visits back there for my money. Eventually I posted a letter threatening legal action. 2 days later i received a cheque in the post, minus the payment for the radiator installation.
I'm more cautious these days of what I commit to when doing any job. I always start off by explaining that any defects found or likely to be found prior to commencing work will be chargeable as extra.
I also note in more detail the client's personality because sometimes you can get a feel for how they are going to be. In my experience the wealthier they are the more likely they will complain about costs.