From cultural taboos to laws ... there are a lots of things a traveller should not do. Like flipping the fish over while cruising on the Yangtze. Bad luck. Big faux-pas. Most thing will be excused (unless you walk into a mosque in a bikini, for instance), but they will leave a bad taste. Or be plain dangerous.
In Ireland, for instance, it is a recipe for disaster if you forget to drive on the left. And it can cost you if you drink in public places. And mentioning the civil war in a dewy-eyed way will raise eyebrows and hackles in most quarters. So ... know the things not to do in Ireland.
And should you be heading somewhere else ... have a look at the comprehensive list of things not to do worldwide, collected by Susan Breslow Sardone. That should keep you on the straight and narrow ...