I am being billed for a hospital visit from two years ago, with the bill simply stating "administrative adjustment" processed just last month.

I am quite familiar with this procedure as I have clients who do this quite often.  What likely happened is the hospital billed you for your co-pay and billed the insurance company for the balance.  Insurance companies send money to hospitals in bulk and it takes time to parse which money is credited to which account (yes, years).  What may have happened is that the insurance company paid less than what the hospital billed, for a possibility of a slew of reasons. Perhaps you hadn't reached your deductible, perhaps you exceeded maximum coverage for your plan, perhaps your plan covered only a percentage of the charges, perhaps the rate your plan covered was less than the rate charged, perhaps they were not participating providers or the contract terminated ... What I would suggest you do is contact your insurance carrier and research the status of the claim. You can request what is called an "Explanation of Benefits" which may very well detail precisely why the bill was not paid in full and why the hospital made an "administrative adjustment."  You very likely signed something at the hospital that stated that you would be responsible for any payments not covered by insurance.  As such, I suggest that you make the effort to discover the basis for the adjusted bill.  You will not likely be sued for such a small amount, but it may negatively affect your credit.