Trail etiquette: What is the #1 problem?

Don't be THAT guy (or girl) on the trail. Tell us the #1 behavior that should be avoided.
A wooden sign attached to a tree in a forest, reading "WRONG WAY FOR TRAIL FLOW," with a symbol indicating a prohibition. The background features a wooded path surrounded by green leaves and scattered stones on the forest floor. Rockland Preserve mountain bike trail.
File photo: Singletracks member Rider51.

Etiquette. Rules. Social norms. Whatever you call them, trail users have developed a set of written and unwritten dos and don’ts to keep the peace and protect the environment. If you’ve been riding mountain bikes for any amount of time, you know that not everyone is aware of the rules, and even those who are aware don’t always follow the rules.

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What is the #1 problem when it comes to trail etiquette?

What is the best way to educate trail users about general etiquette and/or one of these issues in particular? Share your suggestions in the comments below!