Lineup etiquette
What is a drop-in in surfing?
Taking off on a wave someone else already has priority on. The cardinal sin of the lineup.
A drop-in is when you take off on a wave that another surfer is already riding or already paddling into with priority. Even if you did it by mistake, kick out immediately, paddle back, and apologize. Repeat drop-ins get you a reputation nobody wants. When in doubt, look left and right before you commit.