On the subject of non paying clients, of course you would never hand over the code until you are paid in full, and of course getting paid in full is dependent on having provided the client with exactly what they wanted as per your agreement and without genuine faults in the system or programming.

Assuming the system is tip top and you have a client that just does not want to pay, how many of you build "encouragement to pay" into the system? I remember we had a client that would constantly delay payment until he was "satisfied" with the system and we bent over backwards for this guy. There was nothing wrong with his system and we made several visits that were well above and beyond the scope of the system to get the final cheque. We realised that he would never be "satisfied" and just didn't want to pay, so on a visit to make some bullshit change he wanted, we made a few changes that made life uncomfortable for him and his family including lighting operation, HVAC control and distributed audio. All of which started behaving in an annoying manner about a week after that visit (we allowed that week delay to see if we would get paid). He was "outraged" that we would not return to fix this issue until we were paid, but after a few nights of trying to sleep with the bedroom lights on full in an overly warm bedroom he got the message pretty quickly and relented. Amazingly all issues disappeared within 5 minutes of my arrival to pick up the cheque. As i was leaving i mentioned that in about 10 days the system would ask for a once only key (pin number) to be entered to continue normal operation. This was bullshit to make sure the cheque didn't bounce.

Since then the contract got changed to include that the program was a trial version that would expire 30 days after the completion of the job if not paid in full and customers who are unlikely to pay start getting messages on their screens when that 30 day time period is approaching warning of impending shutdown.