There's been a new rule instituted on what is required for a goal. The ball no longer has to just cross the goal line, but it has to make it at least a full yard past the line. Also, this rule only applies to AC Milan.
Either that new rule is a thing or the linesman at the Milan vs. Juventus match made a massive mistake. Yeah, probably the latter. Sulley Muntari's header crossed the goal line and was a good two feet past the line when Gianluigi Buffon knocked it back into play, which was enough for the linesman to somehow rule that the ball had never crossed the line.
Credit to Juventus, though, for everyone on the team being completely convinced that the ball never crossed the line. Just look at how quickly they react and are sure it was not a goal. That's not acting. For one linesman to get it wrong is unfortunate, but for an entire team of 11 to do it is astounding.