Spurred on by the passionate local crowd, referee Angus Gardner not only decided Williams' foul deserved a yellow card, but as he was the only defender in the vicinity of Anthony Belleau's cross-field kick that it warranted a penalty try. Immediately, momentum swung in France's favour.
Amazing how one instant can have such a profound impact on the entire match.
All of a sudden, the All Blacks went from being totally in control to ragged and under serious pressure on their own line.
They went from running in four converted tries at a canter to flying up on defence and desperately winning turnovers at the breakdown.
They went from making slick hands plays to dropping simple passes and losing lineouts with the limited second half possession they did have.
Interestingly, at the same time Williams took a seat the All Blacks decided to replace captain Kieran Read, shifting Sam Cane to No 8 and leaving Beauden Barrett in charge.
Barrett was brilliant in the first spell. He hit the ball at pace off Aaron Smith's quick pass. He challenged the line; varied his options with one pin-point chip in behind for Rieko Ioane. He laid on Waisake Naholo's first try with a pitch-perfect cutout ball and his goal kicking was classy, too.
Initially at least it was an example of how damaging Barrett can be when given far too much time and space.
But after the Williams mistake he, too, struggled to assert himself in the same way. Not a great surprise given the All Blacks were forced to defend for large periods.
The shame of Williams' error was, prior to that, he had some brilliant touches. He laid on Ryan Crotty's try with a superb grubber. As he has all season, his defence was world-class. And he carried strongly on a number of occasions too.
But, no doubt, much of that perspective will be lost in the mist of the frustration around his snap decision that put the All Blacks on the backfoot, and completely changed the complexion of this test.
It is, after all, not the first time this year an incident involving Williams cost the All Blacks.
On paper, this was not one of the great French teams. They were decimated by injuries, handing out four debuts in the starting team and three more on the bench. Halfback Antoine Dupont, in his first start, delivered sharply and made an real impression with his pacey running game.
The All Blacks could easily have nailed two more first half tries but for poor execution and decision-making. And from their commanding position at half time, they really should have put the foot on the throat. Ultimately, it was a patchy performance - a statement all too frequent this year.
The All Blacks, and the spectacle in general, weren't helped by a series of shambolic scrums. Eventually, not before time, Gardner binned French tighthead Rabah Slimani, but then the tables turned and the All Blacks had their issues.
They will also be worried about a knee injury to Dane Coles, who departed mid-way through the first half.
Coming off a loss to the Wallabies in Brisbane this first northern tour test had all the makings of a statement performance. In the end the All Blacks showed character to get out of a tough spot, with Naholo finishing the job.
But from the position they were in, it never should have been this tense.
