Eating with your hands
Burgers might have spread around the world but chowing down on one with both hands certainly hasn’t. Don’t eat anything with your left hand in Muslim countries or in India. That part of the body is used for an entirely different function in such places, one people don’t want to be reminded of when eating!
The left hand really is a poor relation of the right in these parts of the world; you should also avoid gesturing or shaking hands with it. As a curious variation, if their right hand is wet or dirty Senegalese people will offer their right wrist for a handshake or their left hand, but with an apology.
Clearly, if you’re left-handed and want to travel widely, it may be time to retrain.