Faking an injury or overblowing contact so that it looks more violent to draw a penalty.
It's getting really bad in hockey too, I'd like to see more leagues cracking down on it. It's sometimes dubious but it's often a lot more clear nowadays with full HD video from 1000 angles at every game.
Just insta-red card them whenever they dive. Harshness is sometimes the best answer.
You don't want to do that, too many will end up off because the refs got it wrong. Diving shouldn't be handled by the refs but by a panel after the match.