Most amateurs don’t realize that their carefree tap-in could potentially be breaking a rule and subject to a penalty. Under Rule 10c, a player “must not make a stroke from a stance with a foot deliberately placed on each side of, or with either foot deliberately touching, the line of play or an extension of that line behind the ball.”
That means that if you reach across the hole and rake a putt in, you’re breaking a rule and could be hit with a penalty. Similarly, you can’t straddle your line and knock the putt in croquet-style. The obvious exception to this rule is one we see often on Tour: there is no penalty if this stance is taken accidentally or in order to avoid another player’s line of play.