It's very surprising how many domains accidentally expire...

Each registrar has different procedures, but the basic premise is, if the domain registration isn't paid up, the domain will go back on the market, and if you still need it after it is expired for a short period of time, will likely cost you more than it should to retain.  Registrars will email you an invoice, but they can't find you if you've changed your email address or the person that registered the domain no longer works for you.  Organizations need to keep track of their domain renewal dates in house as well, just in case.