Gunners find new ways to hit depths of despair
Arsenal 1-2 Swansea City
Of all the ways that Arsene Wenger's Arsenal of the last decade have conspired to crush the dreams of their support, this defeat at home to a second-string Swansea City team in the white heat of one of the closest title races in many seasons will be one they talk about for years.
Perhaps Arsenal will go on to win the Premier League in this remarkable season of 2015-16 when every challenger seems more consumed with vulnerabilities than strengths, and let us not forget this was a night when both Tottenham and Manchester City lost.
Even so, there was something so poignantly in keeping with the second half of the Wenger era about this defeat, not least in the way in which it so brutally replaced hope with doubt and finished it with the sight of Petr Cech clutching his injured leg after going forward for an injury-time corner which will rule him out of Saturday's North London derby.
Swansea had all but given this game up, with six changes from the team that lost to Spurs at the weekend, presumably in preparation for relegation-critical fixtures against Norwich, Bournemouth and Aston Villa.
They began the game with low expectations and yet it was Arsenal who, in their own failure to put the game out of sight, after Joel Campbell's opener, simply lit the fire of belief in their opponents.
The winner came from captain Ashley Williams, and the introduction of Gylfi Sigurdsson at half-time made a difference, but even so, it was a failure of monstrous proportions.
Was defeat by Manchester United at Old Trafford the turning point? Who will keep their nerve when the two flawed title-hunters of north London meet on Saturday?
The changes were wide-ranging and made by Swansea's absent manager Francesco Guidolin, who picked the team from hospital, where he is being treated for a chest infection.
The caretaker extraordinaire Alan Curtis was back in charge for Swansea.
Arsenal were excellent in the early stages when Alexis Sanchez hit the post you wondered how long Swansea might survive.
One move in particular down the right involved Campbell, Mesut Ozil, and then Hector Bellerin on the overlap with possession preserved judiciously until the ball finally reached the feet of Olivier Giroud.
He was back in the side at the expense of Theo Walcott and the French centre-forward had one of the first halves where he was obliged to strike a few of those disbelieving poses when chances went begging. He lashed one against the bar four minutes before the break when Per Mertesacker generously headed the ball backwards and down for him.
For all the chances they missed, Arsenal's goal was a moment of the highest quality that started with a ball out to the right to Bellerin from Sanchez, deep inside in his own. When it reached Campbell his first shot was blocked but he won it back and it went on to Francis Coquelin who lunged in time to get the ball back out left to Sanchez, running from deep to join the attack.
Sanchez's ball into the area was perceptive but fast and skiddy and Campbell had only one option: he had to take it first time and he had to go to ground in the act of doing so, hitting the shot on the half-volley and on the slide. All in, a glorious goal and it promised many more.
That they did not come in the first half was a testament to the performance of Swansea with their five-man midfield and their reliance on hitting big Bafetimbi Gomis with the long ball to get them out of trouble. Not that the respite lasted long when they were afforded some: Arsenal kept coming back at them and it required a fine defensive performance from Ashley Williams and Jordi Amat in the centre.
Amat's challenge on Ozil was the beginning of the Swansea equaliser on 33 minutes when he won a tackle from a goal-kick that Wenger was convinced was a foul. The Arsenal manager might be equally concerned about the consequences, in which Jack Cork pulled away from Aaron Ramsey and slipped the ball through Arsenal's back four for Routledge to score.
A nice finish from the Swansea man and then it was back to defending for their lives while Wenger flapped his arms on the touchline.
Very soon though, among the Emirates crowd at least, it became a rage against the enemy within. Sanchez could not pass the ball. Ozil tripped up trying to volley one that dropped over his shoulder.
What should have been a routine victory had turned into something more profound, another examination of Arsenal's soul.
Could they do it? Could they take advantage of a very beatable opponent on a night when the title race news from elsewhere had been hitherto positive.
As for Swansea, their interest in the game grew, especially when Curtis took the chance to introduce one of his A-listers, Sigurdsson, at half-time for Ki Sung-Yeung. The midfielder added an extra dimension to the Swansea midfield and on 65 minutes he very nearly scored.
Meanwhile, at the other end Sanchez hit the bar with a free-kick and Wenger was given an instant verdict on replacing Campbell with Danny Welbeck. The home fans did not like it. They wanted Giroud off and they did not mind saying so. But much worse was to come for them.
That was Swansea's second goal, forced in at the back post by Williams when Cech came for Sigurdsson's corner and missed it.
Arsenal struggled on with a penalty appeal in the 90th minute but as the home crowd gradually left, they could not find that goal which might change the night, and all their fortunes.