The One Thing You Forgot To Bring For Your Overseas Trip

Very often, when I go for vacations or conferences overseas, there is always that one thing I forgot to pack. And the epiphany always happens the moment when I'm all belted into the airplane seat. I've forgotten things like the power adapter cable of my notebook, the battery of my camera, even underwear! I'm sure you've had your own moment too. Tell me what's the one thing you wanted to bring on your overseas trip, but ultimately forgot...