What the hell? In Brazil, the "OK" gesture is roughly equivalent to the finger in the US, which means you should not use it when your hotel manager asks you how your room is, unless you want to tell him that it's purple and velvety and recently molested his wife.
The most famous incident of a misapplied "OK" sign was, in fact, Nixon's visit to Brazil in the '50s. While alighting from the aircraft, he lifted both hands to the cameras and double-fingered the entire nation. Nixon went on to greet the Brazilian Prime minister with a savage kick to the testicles, and concluded his visit by urinating from the window of a moving limousine.
If you're visiting Brazil, you should also never touch any food with your fingers. Even stuff like pizzas and burgers should be eaten with a knife and fork. Not that you'll ever need to apply this knowledge, because after reading this article, you'd be insane if you ever travel abroad again.