Sometimes in a game with friends one of them may be in a hurry to make his putt, so he removes the flagstick, laying it down on the green too close to the hole. As he's doing this, another player makes his putt and the ball runs past the hole and is about to be stopped by the flagstick. So the first player quickly picks the flagstick up and puts it far out of the line of play. Why is this a problem?
In the first place, the flagstick should never be placed on the green. It should be put on the edge of the green for two reasons: so it won't interfere with play and so it won't damage the grass surface of the green. Also, according to USGA Rules, if the golfers are involved in matchplay, the player who removed the flagstick would automatically lose the hole because he removed an obstruction. In stroke play, he would get a 2-stroke penalty. However, there would be no penalties for the second player.