A huge red flag you say?

In the US and Canada at least, there are laws that cover granting birthright citizenship to people born in their airspace of those countries. And since they share a border, it could happen, in theory. Would also depend on the citizenship status of the parents, I imagine…

In practice, I would hope you are right, this would not cause any issues. But if it happened, it would probably get news attention, and who knows what follows from that.

Twins born on an airplane in flight could be born in different countries.