Just because a hand gesture is safe to do in the United States doesn't mean it's safe to do in foreign countries. Doing any one of these normally innocent gestures in the wrong place could start a fight or even land you in jail!
While crossing your fingers is a good luck charm in the United States, you probably don't want to do the same gesture in Vietnam. In that country's culture, it represents a woman's private parts. Showing this sign to someone is a massive insult that's essentially the same as called them the c-word.
Sticking out your index and pinky fingers to make the shape of horns is something you'll see at just about every rock concert ever. It basically just means "rock on". However, don't ever flash the horns if you are visiting Spain, Italy, Portugal, Argentina, Brazil, or Greece. In those countries, the gesture is known as the "corna", and doing so is an insult that implies a man's wife is cheating on him.
Depending on which country you're in, making the OK symbol with your hand can have a very different reaction. In the United States, it simply means "OK" and it even resembles the shape of the letters. In Brazil, it's basically the same thing as saying "f*** you". In Greece and Turkey, you're signifying that you're a homosexual.
The thumbs up is the universal sign of approval in America, but things are a little different down under. In Australia, this symbol means "up yours" because you're essentially telling someone to stick something up their rear end. It's even more of an insult if you motion your hand upward while making the thumbs up.
Come Over Wave
In many less-developed areas of the world where soap and toilet paper are less common, the left hand is still reserved for taking care of cleaning up after yourself on the toilet. Even if you're left-handed, make sure to offer your right hand to shake. Offering your left hand is an insult that's basically the same as handing them a pile of poo.
While slapping your hand over the top of your fist isn't exactly a common gesture, it's the kind of thing we do without thinking. In France, this gesture means that you want to fight the person it's aimed toward, which is the exact reverse as the American gesture that means the same thing. If you happen to make this sign in Chile, it's even worse -- it means they have a small package.
Luckily, this isn't something you would do on accident, but making the five fathers gesture is pretty much the worst thing you can do in Saudi Arabia. By putting your index finger to the three main fingers on your other hand essentially means "your mother is a whore". The symbol represents one woman having so many sexual partners that it's impossible to determine who the father was.
Waving your hand back toward your face is a very intense way of telling someone to come closer in the US. In the Philippines, using this gesture can actually land you in jail. You're basically saying that that person deserves to die, and it's typically only used against stray dogs. Many countries use the upside-down version of this gesture instead because they feel it's more poilte.