One of the challenges for referees is that the human eye can handle only approximately 16 images per second, which means the ball needs to be behind the line for at least 60 milliseconds.
However, in some cases the ball is only behind the line for a few milliseconds before a player kicks it back or it rebounds back into the field of play, with the result that the human eye cannot see whether the ball has crossed the line. The ball can only be detected by the human eye at a speed of 12km/h or less, whereas nowadays players are able to shoot at a speed of over 120km/h (cf. “The hardest recorded shot in football – ever”, 14 February 2007, The Guardian).
Another challenge is the vantage point. When viewed from certain angles, it is easy to misjudge the ball’s position. Cameras placed at different angles can mislead viewers when showing images “proving” whether or not the ball has crossed the line, which is why only technology dedicated to evaluating such incidents can support the referee in the decision-making process and contribute to a fair game.