If someone crosses the line they start behaving in a way that is not socially acceptable.
A change from being acceptable to being unacceptable.
To do something wrong.
1. The hockey players had crossed the line when they started a brawl with their fans.
2. If you steal someone’s idea for your own, you have definitely crossed the line.
3. Sometimes stand-up comedy can cross the line and be offensive.
4. You crossed the line when you lied to me.
bite off too much, go overboard, go too far, not know when to stop, overdo, stretch