This is one of the most common mistakes I get every time. I hear my friends travel around and get into problems because their travel document is expired. This is a very expensive ‘joke’.
Customs don’t care anything about your passport and valid date. You have to learn it the hard way by canceling your trip and paying the entire price for it. And.. on top of that, you have to pay for a new ticket. Ah it’s just a mess. And it’s kind of weird as well, as if you don’t longer look like the person in your passport photo.