Airline? Departure City? Routing? Length of Stay in Thailand?

Some responders mention the carrier is UA, so they definitely check for a valid visa for the length of stay. This is a non-issue if the length of stay is 30 days or less (U.S. passport holders get 30 days visa exemption). Further they must check the validity date of your passport to make sure it is not expired. But I do not know for sure how closely they look at the remaining validity vs. the requirements of the country to be visited. If its automated then you're out of luck. Checking in on-line really means nothing in this case, as you will be directed to an agent. At the other end (Thailand: entry and exit) you will be subject to the uniqueness of the Thai Immigration service. They love paperwork, and will document the date (auto-scanned) of the passport which may pop an automatic flag, and enforcing rules haphazardly.