I think a great deal depends on the nature of the service. If you have been getting, let's say, free cable TV for a year simply because they were too incompetent to bill you, I think you can drop it any time you like and they are pretty much SOL if they then realize they haven't billed you. They are a private entity (I think?) and responsible for their own business affairs. If on the other hand you have been getting some sort of publicly provided/regulated utilities (like electricity
or phone or something) and you decided to perhaps drop the land line in favor of exclusive cell phone service they might well be able to get after you in small claims court for any back billing even if it was "their fault" for not remembering to bill you. Public regulation is a double-edged sword in some ways. It protects you from exhorbitant rates, but it also saddles you with some additional responsibilities and they might be able to hold you to those rather than have to live with their mistakes.