I work claims in an insurance company, and believe me, this happens all the time, for several reasons. Most commonly, the provider forgot to bill, or the claim was somehow lost in transit and they forgot to follow up to see why we didn't pay. Unfortunately, we follow the 'one year timely filing' rule, so at that point you have to fight it out with the provider, unless there is a proper paper trail showing that they did TRY to bill.
I also see where sometimes we go back through the files and audit compliance, and once in a while an old claim will get uncovered and processed. This is very rare, but does happen, or the provider will come back after a few years and try to refight an old denied claim. Sometimes they win!
It also happens that medicare/medicaid will bill us years after the fact, so an old EOB might pop up, causing vast confusion to the insured, who can't understand why on earth we processed such an old claim.
Advice? Call the insurance company first, and find out why they are just now getting around to processing this claim. And, don't pay anything to the provider unless they actually send you a bill, even then, question them on why you owe this after so long. It may well be that this is a duplicate claim, and it was already paid 3 years ago.
As for the folks above complaining about primary and secondary coverage, I have fought this battle many times, and it sucks. There are so many variations, from divorce decrees, to effective dates, to group vs individual coverage, that getting it straight is a real chore. I have even had a few cases where we insisted we were primary, and paid in full, and so did the other company, so the provider got double paid, which is illegal. After months of trying to work it out, my top manager finally just had me tell the provider to refund the difference to the insured, since neither we nor the other insurance company was willing to take the money back! Still not right, but at least the provider's books wouldn't show an illegal balance if they got audited!