Following the practice in the place that you're in: 3 examples, including a certain kind of wide shoes. As discovered by those who violate those norms, and how they conduct themselves afterwards. Also: Another such example, specifically a difference between the Land of Israel and Babylonia, as per Rabbah bar bar Chanah's experience - and how he conducts himself (another few examples). [Who's Who: The Kutim/ Samaritans.] With some observations about the cases the Gemara discusses.