What are some unwritten social rules everyone should know? - Questions & Curiosities

The top ten unwritten rules in my list:

1. Don’t call someone more than twice continuously. If they don’t pick up your call that mean they have something more important to attend to.
2. When your fellow colleague/ classmate/ team-mate gets shouted at please don’t stare at them. It makes the moment twice awkward.
3. When someone drops something on the floor by mistake or drops food from the plate or doesn’t know how to use a knife/fork don’t stare at them. The same goes to people sneezing, coughing or even an uncontrollable fart. It’s an involuntary reaction.
4. Always skip using the washroom beside the occupied one. It makes it uneasy for the person in the occupied washroom as well as yourself if you occupy the one right next to theirs.
5. Return money that you have borrowed even before the other person remembers lending it to you. Be it $1 or $100. It shows your integrity and character. Same goes with umbrellas, pens and lunch boxes.
6. Never order the expensive dish on the menu when someone is treating you for lunch/dinner. If possible ask them to order their choice of food for you.
7. Don’t ask awkward questions like ‘Oh so you aren’t married yet?’ Or ‘Don’t you have kids’ or ‘Why didn’t you buy a house?’ For god’s sake it isn’t your problem.
8. When someone makes a wrong investment and loses their money, don’t add fuel to the flames by saying ‘I knew this would happen. You should have listened to me’. Don’t make failure seem worse for anyone.
9. Always open the door for the person coming behind you. Doesn’t matter if it is a guy or a girl. You don’t grow small by treating someone well in public.
10. Wear ear phones when traveling on a subway or bus. Don’t make others taste your choice of good/bad music.

Edit: The most important one! Don’t swipe left or right when someone hands over their phone to you for viewing the picture. And also don’t browse through their messages and call log list. Thanks!