Join Date: May 2000
Location: Houston, TX, USA
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Posts: 7,970
There's one other thing that occurs to me: you said that the VCE-xxx-ATH segments were on the same carrier, whose hub is at xxx. Were both of those segments on the same ticket? Were you given boarding passes for both segments at VCE? If is, it is a bit unusual for the airline not to catch this at that point. I would have no expectation that they'd catch this prior to departure at SIN. But as to each portion of the journey, I'd expect them to check documents at the point of check-in, including all segments for which boarding passes are being issued.
Note that I'm not saying that they have an obligation to the passenger to do so, but just that that would be my understanding of standard procedure.