What Unwritten Rule Should Everyone Live By?

What’s an unwritten rule you think everyone should follow?
This conversation was started on reddit. People chimed in on the thread to share things that aren’t written down or enforced but, maybe should be. Here’s some highlights.
If someone hands you their phone to show you a picture, don’t look at other photos. Don’t heat up fish in the office kitchen, or anything else that smells. Don’t propose at a wedding. If you borrow someone’s car, fill it with gas. Or at least get it back to where it was. If you cancel on a friend, it’s your responsibility to reschedule.