1. Believing your passport is the key to the galaxy
Certain countries (including Russia and China) require not only a passport, but a passport that is valid six months past the date of your return flight. Perhaps this keep foreigners from languishing in their jails when their passports have expired? 26 other European nations require your passport to be valid three months past your departure date.
Another small reminder, your passport is not the same as a visa. Make sure you don’t need the latter before traveling to some faraway place only to promptly be sent home.