Six months' validity beyond your return date. Not six months from today — six months from the day you fly home. Every year we catch at least one.
It is one of the most avoidable reasons a trip falls apart before it starts.
Most countries want your passport to be valid for at least six months beyond the date you intend to leave their territory. Not six months from the day you book. Not six months from the day you fly out. Six months from the day you come home.
So if you are travelling in December and your passport expires in March, you are inside the window and you will likely be refused boarding — by the airline, at check-in, before you ever reach an immigration desk.
A few more that come up:
Blank pages. Some destinations want two consecutive blank pages, and a well-travelled passport can run out.
Name matching. The name on your booking must match the name in your passport exactly. If you have married since you last travelled, tell us before we ticket, not after.
Children's passports. They are usually issued for shorter periods than adult ones, and parents forget.
Every proposal we send checks the current entry requirements for the passport you are actually travelling on, and we check again before you depart. But your passport's expiry date is something you can look at right now, in about ten seconds. Go and do it.