It's one of the 10 commandments of surfing, with violations often called the sport's cardinal sin: Never, ever, drop in on a fellow surfer, stealing the wave.
But as Australian professional Soli Bailey found out this week, some locals play by their own rules.
As Surfer Magazine reports, Bailey was "already feeling a bit shaky" while surfing in the shark-infested waters of Southern Australia.
Lucky for him, the dark figure that appeared as he emerged from a left-handed barrel wasn't a shark, but a much friendlier type of local.
Baily, 20, as seen in the video below, narrowly avoided a collision, successfully riding out the wave with an electrified look on his face.
Had the drop-in been by another human surfer, Bailey may not have been so nice. But how often does one get to go fin-for-fin with a dolphin?