1. Fall asleep.
2. Insult people in their native languages.
3. Insult people in your own language. The people you are insulting can understand you. They just don't want you to know. Stop being such an idiot. I do not feel like socking you on the tram, thanks.
4. Perform public displays of affection. I saw this one on the tram today, on the part of a group that shall heretoforth be referred to as the two-headed monster. There was actual head-in-lap action. I'm pretty sure that's not kosher anywhere. Even in Belgium.
5. Laugh at alcoholics. Yes, it's templting, because they sip their beer with such gusto, but please retain some sense of decency.