Scolari faces up to home truths
Chelsea winning at home has ranked alongside death and taxes in the list of life's certainties in recent seasons - but Arsenal's victory showed how times have changed at Stamford Bridge.
It used to be almost impossible for any team to lay a glove on Chelsea on home turf in the Premier League in an unbeaten run stretching back over four years until Liverpool's victory in October.
But Luiz Felipe Scolari's agitated performance during and after this game was a reflection on how Chelsea had been floored by two classic sucker punches from Robin van Persie.
Indeed, the body language of Scolari and his Arsenal counterpart Arsene Wenger told much of the story of a game that looked to be moving towards a routine Chelsea win until the Gunners hit back to suggest it may yet be too soon to pen their title obituaries.
Scolari was a picture of animation in the post-match inquest as he demanded an apology from the match officials after they missed a clear offside in the build up to Van Persie's crucial equaliser.
It is almost impossible to dislike Scolari, and while he was clearly boiling inside, it was also hard not to sympathise with the frustration he felt.
But when his blood pressure drops and his understandable sense of injustice subsides, he will know Chelsea's current problems run deeper than a linesman's failure to raise his flag.
It was a defining moment, but more worrying for Scolari was Chelsea's lack of response and ideas once they went behind. As against Liverpool, the visitors looked more likely to add another goal than Chelsea did to get an equaliser.
Chelsea have a meagre return of one point from three home games against their main title rivals this season, and the armour of invincibilty they used to carry at Stamford Bridge has been stripped away.
If you require any further evidence, three wins in eight league games at home provides it, although it is hardly crisis time.
Chelsea still top the Premier League, at least until Liverpool play West Ham on Monday, and the jeers at the final whistle were the reaction of a minority of supporters who desperately need a reality check.
Deco was another who felt the force of the terrace taunts, with ironic cheers greeting his second-half substitution.
The Portugal midfield man has faded since the sunny days of August. He was slack in possession, off the pace and lightweight in the physical challenges.
Chelsea cried out for Joe Cole's optimism and invention, the record showing that they are currently lacking the spark to break down stubborn teams on their own turf.
And, like him or loathe him, the suspended Didier Drogba gives them a powerful weapon and he was also badly missed in the closing stages as Arsenal mopped up a series of predictable attacks.
Chelsea are hardly in meltdown. They remain title favourites and the bookies will not be changing those odds just yet.
But as doubts appear at Chelsea where previously there were none, it will give encouragement to Liverpool and Manchester United that they can take advantage of this sudden fallibility.
As for Arsenal, it would appear they are intent on ignoring the form book as they beat the big names and lose to the lesser lights.
Wenger was as animated as I have ever seen him in the first half at Stamford Bridge. It was a close-up view of a manager who knew he could not countenance defeat.
He berated the fourth official and on one occasion just turned away with a gritted-teeth grimace when a pass failed to find its target. Like Arsenal's title challenge, Wenger was right on the edge.
I asked the Arsenal manager how important the victory was in the context of their season, and he made no attempt to hide its importance.
"A massive win. We knew we had just lost two league games and to lose again would have put us 13 points adrift and that would have been too much. We showed we have great character here.
"I am very, very happy. We have beaten Manchester United and Chelsea. It proves I have an intelligent and talented team."
The follow-up question was obvious. Does this not make defeats against teams like Hull, Stoke and Fulham more frustrating?
"That's true, but we have also shown we have the needed level to be there and it is now down to our desire to win in the coming months."
No-one could question the desire at Stamford Bridge, although the performance of Emmanuel Adebayor was a sharp contrast to the industry and threat of Van Persie.
Adebayor livened up once Arsenal levelled, but he was a source of obvious frustration for the visiting fans until that moment. Wenger needs him to be in the groove from the first whistle, not sparking to life once the going is good.
Wenger has been given the nickname of The Professor and he will be using all his cerebral qualities to ensure Arsenal do not slip mentally in what they perceive to be lesser challenges.
For Scolari, the trick is to re-create that old invincibility and give Stamford Bridge that fortress feel it has lacked against Liverpool and Arsenal.