I'm in New Zealand, but I'd be happy to hear an answer for your jurisdiction too.
Due to a glitch in a billing system a single client was billed for some monthly service zero dollars. That client has been receiving the service, but he did not pay for it. He obviously knew how much the service should have cost when he signed up, and there were some months when he was billed and paid appropriately before the glitch happened. The glitch was noticed by the service provider after some months. The client did not attempt to contact the service provider during this time to inquire why his bill changed.
Is it possible to get a decision (in the framework of law) that this client owes this money, or does, from legal perspective, it's company responsibility to provide correct bills in order to be paid appropriately.
It is of course possible to ask him nicely, and hope he agrees to pay, but if not, who is in the right? The amount we are talking about is quite small, around 400USD.