Originally Posted by
KLouis
You're absolutely correct, but the opposite is also true (intra-Schengen flight: my sister was denied boarding because her Schengen-country passport (!) had expired three days earlier). I guess there are explicit rules somewhere, but a) there are strict check-in agents, b) there are laissez-faire check-in agents and, unfortunately, c) there are stupid check-in agents; in brief you'll hear all possible outcomes from travellers, frequent or not, who faced situations.
Your sister's example is an example of an expired passport and is quite distinct from a situation of having a valid passport with expired visas, expired residence stamp/sticker, expired residence cards, or stamp-situation indicating possible overstay.
I like the characterization of how it goes with check-in agents. And yes there are indeed various possible outcomes, some outcomes far more probable than others and varying based on the involved factors.