Faking injuries. Basketball players are the worse at this. Football players can be the second worse. At least in football they have to come out for 1 play, but sometimes the injury is so minor and the player probably would've come out of the series anyway that they don't even go get evaluated on the sideline. Change this to the player has to come out for the series, or a set amount of time (12/15 minutes), and you'll see most of these fakes go away.
Baseball players, when they get injured, are usually injured. The less serious injuries involves a few minutes of stoppage, but usually doesn't benefit either side.
Hockey players, when injured, usually leave the game, are declared dead, get 170 stiches for massive loss of blood, receive an organ or limb transplant, and make it back to the bench to be on the ice for their next shift.