"Seriously, if you take that picture, I'm going to put a hex on you."
How many times have you been in the situation where you literally want to rip a camera out of someone's hands so hard that you're willing to sacrifice grace, dignity, and your physical well-being in exchange for a tiny electronic device? This tends to become an issue the morning after an inebriated evening. You wake up all jolly, just so excited to see all of the great memories that you've captured from the previous night's festivities, and then there it is. That first picture forever freezing you in all of your glory. And then comes the rationalization: But at the time, I definitely WAS a sexy vixen, right...Mom?
While there are many a moment where I've certainly regretted jumping into a polaroid or allowing friends to take a "hilarious" [read: humiliating] video of me in action, there were definitely measures I could have taken to prevent the inevitable [embarrassment] from being both recorded & released by the PapaRazzi. Additionally, while here in the good ole US of A, we just love to razz one another for a good laugh at each other's expense, it can be a very different story abroad.
Regardless of where you may be traveling, it's absolutely vital to ask permission before photographing anything, especially people. While photography certainly boasts a plethora of advantages including sharing the world with others & capturing memories, it can be viewed as intrusive by those that know nothing about you. Asking permission is just a common courtesy that is well received anywhere - it helps to boost confidence & to connect with others. Believe it or not, in some places, taking pictures of people translates to stealing their souls. Unsolicited photography may also render people inferior, as if they're just another monument, piece of scenery, or animal. It is imperative to be open-minded when it comes to photographing abroad, and to thank your subject whether s/he allows you to take the picture or not. Additionally, don't be too hasty to photograph in international airports. Just follow the literal interpretation of these two words, "Ask" "Permission", and you should be golden.
For more specific information on this subject, Darren Rowse provides solid advice in his post "Asking Permission to Photograph People".