It's one of the great mysteries of soccer: In a sport where the objective is to score goals, why do we put up with needless uncertainty about whether the ball has actually crossed the line? Why doesn't soccer use the technological tools at its disposal to objectively determine whether the very raison d'être of the game has actually taken place?
Soccer has a long history of so-called ghost goals. These are situations in which the ball crosses the line and comes back out, but the officials fail to award the score. Usually this happens when it's cleared by the goalkeeper or a defender on or behind the line. Sometimes, a long-range shot will hit the underside of the bar, bounce straight down behind the line and then, heavy with backspin, carom out like a billiard shot. Occasionally the ball does not cross the line but the officials fail to notice and give the goal.
The most famous example of such a disputed goal effectively determined the outcome of a World Cup final. The most recent could decide the Italian league title this year.