Never drop in on another surfer uninvited, it’s against the law. Kinda.
Carve Magazine concisely explain the drop in law and potential consequences, as follows:
“One drop in is an accident, two is rude, three is a twatable offence.”
If you do accidentally drop in; exit at the earliest opportunity and apologise to the surfer you’ve violated.
If you think the drop in law does not apply to you, be prepared for the penalty.
Here’s a surfing Crime and Punishment gallery as evidence:

Beware: droppers may get a board up the butt
Source: Nathan Rupert

Unwanted guests should be shown the exit
Source: MSW

Double trouble
Source: Kewel Teacher/Photobucket

You can run but you can’t hide
Source: Doug Falter/MSW

Never drop in on aerial ninjas
Source: Zac Gibson/MSW

Body surfers do not have priority – head slap penalty maybe applied
Source: Raniom/MSW

Durban droppers get wrestled
Source: GQman

Hawaii oh-no
Source: Surfer today

Airstrikes prove an effect deterrent
Source: Charlie Durrant/MSW
Surfer Dad in no way condones aggressive behaviour, even if someone does drop in on you. In the words of Julie-Ann Burkhart “I like to encourage aloha and also know I was once a beginner. Spread good vibes!”.
eddie would go
eddie would go