Nic Fleming suggests that soccer fans at this month's Confederations Cup in Brazil should recognize that electronic-eye verdicts on goal-line accuracy are not always definitive (Nature 497, 537; 2013). By contrast, American football fans are already well aware of the limitations of replays.

In American football, a call is overturned only if there is indisputable visual evidence from the video review that the on-field call was incorrect. If the replay results are ambiguous, the field call stands.