The customs that trip visitors up are rarely the exotic ones, which guidebooks flag loudly, but the quiet defaults locals never think to mention. The high-frequency categories worth researching for any country: greeting style, since offering the wrong one, hand versus bow versus kiss, creates the first awkwardness of every interaction; shoe etiquette, as removing shoes indoors is mandatory in much of Asia and expected in many homes elsewhere; giving and receiving with the correct hand or both hands, significant across Asia, the Middle East, and Africa; table manners around shared food, utensils, and who starts eating; tipping, which ranges from obligatory to mildly insulting depending on the country; and volume, where the acceptable loudness of public conversation varies more than almost anything else.
The meta-rules that work everywhere: watch what locals do and copy them, err toward formal and modest until you learn the range, accept hospitality graciously since refusing it offends more widely than any faux pas, and apologize with a smile when you slip, which resets almost any moment.
Genuinely rude behavior worldwide is a short universal list: impatience with slowness, mocking local ways, treating service staff badly, and loud entitlement. Avoid those four and individual customs errors get forgiven as endearing rather than offensive, because effort is visible everywhere.