The big four to handle with care almost everywhere: domestic politics, religion, money, and territorial or historical disputes. The last category is the one travellers underestimate; many countries carry sensitivities about borders, wars, colonial history, or neighbouring states, where a casual remark or even a mislabeled map can genuinely offend. Criticising the royal family is not just rude but illegal in a few countries, and mocking religion can be similarly serious.
Money questions cut both directions: in some cultures asking your salary or the price of your possessions is normal friendly curiosity, while volunteering such details elsewhere seems boastful. Personal topics like marriage, children, weight, and age are freely asked in much of Asia and considered intrusive in the West, so expect questions that startle you and take them as friendliness rather than rudeness.
The safe operating mode: ask questions rather than offering opinions, especially on local politics and history; let locals criticise their own country without joining in, a courtesy appreciated worldwide; and steer toward food, family, sport, travel, and your genuine interest in the local culture. Before visiting, skim a culture guide for your specific destination to learn its particular sensitivities, since every country has one or two that outsiders would never guess.