For Manchester City, it was the killer blow and Mario Balotelli made sure there was ignominy attached. A five-point lead has turned into an eight-point deficit in little more than a month and the players' body language at the final whistle told its own story.
Some stood with their hands on hips.Others sank to their knees and when they finally summoned up the energy, the walk to the away end was an apologetic one. They knew it was over.
By that point Balotelli was already in the dressing room, sent off on the back of a dangerous, chaotic, birdbrain performance that makes it unlikely we will see him in City's colours again this season. After that, who knows? For the first time Roberto Mancini said he would "probably" try to sell him and, if it was not just heat‑of‑the‑moment stuff and his patience has truly snapped, he cannot be blamed.
Mancini also said José Mourinho might have been right when he described Balotelli as "unmanageable", and yet City's manager later reiterated his affection for a player he strenuously denied he had overindulged. "I love him as a guy," he said.
All that can be said for certain is that, at this stage of the season, Mancini's team had to emerge with three points from this match and exert some pressure on Manchester United at the top of the table. Instead, City have won only one of their last five games, taking five points from 15.
Their form has deteriorated at just the wrong time and Arsenal fully deserved the win that takes them back into third place, two points clear of Tottenham Hotspur.
Arsène Wenger's team dominated for long spells, struck posts through Robin van Persie and Theo Walcott, missed an open goal twice and could reflect on a number of other chances before the decisive moment, on 87 minutes, when Mikel Arteta dispossessed David Pizarro 40 yards from goal, advanced towards the City penalty area and beat Joe Hart with a right-foot drive. Wenger was correct afterwards when he talked of "consistent domination". With better luck, Arsenal would have won far more convincingly.
Their opponents had been restricted to brief spells when they threatened to get on top. Balotelli had one breakaway chance in the first half and turned in another show of petulance, kicking a post in frustration.
The same player had what every amateur golfer knows as an air‑shot with City's best opportunity of the second half and, that apart, the only time Arsenal's goal was seriously threatened came via a 53rd‑minute cross from Samir Nasri. Sergio Agüero and Pablo Zabaleta got in each other's way and as they looked at one another for an explanation, it felt symptomatic of how City have unravelled.
Nothing, however, epitomises their meltdown more than the contribution of the Italian striker with the mohican, the attitude and the message on his vest saying: "You only live once."
We never got to see that slogan beneath Balotelli's shirt. We did, however, witness all the worst traits of a man who has stayed too young too long. His season has become a portfolio of lunacy and this was arguably the lowest point.
The surprise was that his red card, for a second bookable challenge on Bacary Sagna, took until the final minute of normal time to arrive. The more serious offence was on Alex Song to leave the Arsenal midfielder on the floor after 21 minutes and remind us, quite simply, that Balotelli has gone past the point where there is any real charm and strayed into a place where it is difficult to find any legitimate form of defence.
It was his studs versus his opponent's kneecap, the classic over-the-top leg‑breaker. Song was fortunate he was able to get up and if the referee, Martin Atkinson, tells the Football Association that his view was impeded, as television replays appear to demonstrate, Balotelli could be looking at retrospective punishment for the second time this season. "There are six games left and he won't play," Mancini said, resigned to a six‑match ban.
Ultimately, though, there were plenty more reasons for City to be disappointed. Arsenal always played with the greater momentum, applying strong pressure from the start. Van Persie could reflect on one header that connected with the woodwork and another that struck his team-mate, Thomas Vermaelen, on the goalline. Vermaelen and Yossi Benayoun both missed when it had seemed easier to score after Walcott's low shot skimmed against a post.
At the end City's assistant manager, David Platt, urged the losing players to go over to the away fans. The television cameras had already picked out a young boy in tears. Carlos Tevez, however, had already gone down the tunnel, along with a few others. It was a shabby way to end a horrible day, and the ordeal might become even worse given that Sir Alex Ferguson's 13th title can be confirmed at the Etihad Stadium, on 30 April.