The manner of Manchester City's 3-2 defeat to Norwich was more alarming than the five-point gap it created between the Premier League champions and current leaders Liverpool.
A newly-promoted side, shorn of 11 regulars, were sharper, stronger and braver than the title holders.
The Canaries fully deserved an unexpected victory against an error-strewn City side, who were unrecognisable from the team that has cantered to victories all over England for the past two seasons.
The deficit to Liverpool is, of course, far from insurmountable, even if both clubs have barely made a mistake in the past few months.
But Pep Guardiola must be concerned with a dismal, disjointed performance, which was far more reminiscent of his inconsistent debut season than anything that has gone on since – and every bit as shocking as the details of 'Operation Yellowhammer', the British government's contingency plans for Brexit.
Defensively, they were a disaster. Vincent Kompany went at the end of last season and with Aymeric Laporte out until the New Year with a knee injury, they look short of organisation and leadership.
John Stones is majestic when he's full of confidence – but at Carrow Road he was anything but.
Uncertain in the challenge and nervous with the ball at his feet, the England international looks to be still suffering a hangover from his troubled summer at the Nations League.
Defensive partner Nicolas Otamendi was lax throughout, caught napping for an embarrassing third goal at a time when City were looking to haul themselves back into the game.
Oleksandr Zinchenko struggled to play the simplest of passes as he became the pantomime baddie at a raucous Carrow Road and looked like a makeshift left-back rather than the natural he has become.
Kyle Walker could at least avoid criticism. But it was all very un-City like.
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Norwich on the other hand were superb, playing with passion, energy, intelligence and, most notably of all, courage.
Brave teams are capable of stopping City by denying them possession but they have to be at their best to do so without paying a heavy price.
Brighton tried it at the Etihad Stadium in the game that preceded the international break but got it wrong after just a minute, and from then on, the game was gone.
Norwich were courageous too, despite being so short of numbers that they had to name two goalkeepers on the bench.
Their short, sharp passing broke the hectic press from Bernardo Silva, Ilkay Gundogan and Sergio Aguero, making it hard for the champions to maintain the sort of control to which they are accustomed.
At the back, Norwich were well-organised, squeezing the space between the backline and midfield, where David Silva works his magic, and denying room for Raheem Sterling and Bernardo on the wings.
And when they got their opportunities, they made them count.
Kenny McClean headed them in front with a near-post flick in the 18th minute. The defender took up a space that was eerily similar to the one used by Lucas Moura when he nodded home for Spurs earlier in the season and not unlike Fernando Llorente's goal for the same team in last season's Champions League.
Despite their relative lack of height, City have been well-organised in dead-ball positions for the past two seasons but Guardiola will need to work on a weakness that opponents have clearly identified.
Norwich's second was worthy of anything that the Catalan has taught his side.
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The home side strung together 30 passes as Teemu Pukki broke City's high defensive line before rolling the ball into Todd Cantwell for a tap-in.
City were out of luck when Raheem Sterling smashed a header against the post from close range but they finally got the breakthrough just before half-time when Aguero finally found some space in the box to flick home Bernardo's cross.
The Argentine became just the third player to score in a club’s first five Premier League games of the season, after Arsenal's Jose Antonio Reyes in 2004 and Wayne Rooney for Manchester United in 2011.
But just as City threatened to get themselves back into the game, they gave away a disastrous third.
A sloppy Otamendi was caught dawdling on the ball on the edge of his own area from a back pass by Emiliano Buendia, who teed up Pukki for a simple finish.
Guardiola threw on Kevin De Bruyne, Gabriel Jesus and Riyad Mahrez to try to rescue a point.
Rodri smashed in a 20-yard strike in the 88th minute but Norwich maintained their fight to get the victory over the line.
A memorable night for the Canaries, and for City too – just for all the wrong reasons.