Had two last season, though it should have been three probably.
One was a blatant dive in the box, it was 1-1 but the opposition was down to nine men, so they were throwing the kitchen sink looking for a winner to no avail. I was assessed that day and got the plaudits for the simulation caution on top of the double red.
Second (and third) was in a one-sided match, two different players did very dramatic dives in the middle of the pitch. They were tackled fairly and they were no where near needing to evade or anything, they just swan dived. I cautioned one and left the other because an attack was happening and I ended up losing who it was. That one was just a bizarre day.
And this season I've had one flop-dive in the box, with no one near the guy, but I elected to leave it as they were getting hammered and I was halfway towards wondering whether he lost his balance in the run into the box, so I was being generous and let him off. My AR on the day agreed with the call on game management grounds, but felt I should caution him if the game was a tighter affair, which I can agree with.
Pretty rare incidents though, or perhaps I miss the other ones and am getting conned? Who knows.