Almost everyone has heard: "Your passport needs 6 months validity to travel." True in general — but the actual rule most people get wrong isn't about the 6 months. It's about whose 6 months.

Some countries measure validity from your date of entry. Others measure it from your planned date of exit. A few (a small but real number) actually require 6 months validity beyond your visa's expiry date, not your trip's.

This matters because a passport with, say, 5 months and 20 days left can still get you a visa approved — and then denied boarding at the airport, because airline staff apply the strictest interpretation of the rule to avoid being fined by the destination country for carrying an inadmissible passenger.

The airline, not the embassy, is often your last checkpoint — and they're not always working off the same rulebook as the consulate that approved you.