I once discovered when filling out an arrivals card on a flight that I accidentally brought an expired passport with me. United hadn't checked IDs at any point in the process, and I got my boarding pass on the mobile app using a stored, valid passport. Seemed like a major screw up on their part not to check anyone's travel documents, but anyway it was entirely my fault and all the blame falls on me. Needless to say, I am obsessive about checking to ensure I have all the right documents with me these days.
(Long story short, some strings were pulled and I was let into the destination country anyway, and got a temporary passport issued from my own embassy so I could return home. Actually this was the best outcome; had United turned me away, by the time I got my valid passport and took the next flight, which was the following day, the trip would have been in vain.)