Swansea City 3-1 Arsenal
Arsene Wenger blamed Arsenal's defeat on "unusual, massive mistakes" for which it was "very difficult to find a rational explanation."
Arsenal were well beaten by a Swansea side that started the day at the bottom of the Premier League, despite going ahead in the first-half through Nacho Monreal. But Swansea equalised through Sam Clucas less than a minute later, a goal that Wenger described as the "turning point" of the game.
"We were absolutely guilty in giving the goal away," Wenger said. "We have no rational explanation. Swansea won decisive duals offensively and defensively. On top of that we made unusual, massive mistakes. That was the killer.
"Swansea were sharp, disciplined and hungry. Unfortunately I believe we were not good enough, I believe we were not disciplined enough.
"I don't want to talk about second or third goals."
In trying to explain another limp defeat, Wenger seemed to echo David Moyes's famous words from his time as Manchester United manager, when the now West Ham boss noted that his team could "improve in a number of areas, including passing, creating chances and defending."
"We were not convincing defensively, not convincing offensively," Wenger said, with some understatement. "In the end we lost the game."
Swansea's second goal came from a calamitous error by Petr Cech, after he miss-kicked a clearance straight to Jordan Ayew, who slotted into an empty net. But Wenger seemed more angry about the throw-in that lead to the error: Monreal had the ball deep in Arsenal territory on the left, but instead of passing forwards he elected to let it run out of play.
"There was no need to let the ball go out for the throw in," said Wenger. "We you have a throw in for you, you're 9 vs 10 on the pitch. Why put a handicap like that on you? We could attack straight away."
As he awaits the finalising of his transfer, one wonders what Pierre-Emerick Aubameyang made of it all.
He will doubtless have watched his new team from one of the finest hotel suites in London, anticipating a contract that will be the most generous the club have ever agreed, but this showed why Arsenal are so eager for him to join, and he will also know the scale of the task.
The new Arsenal emerging from the departure of Alexis Sanchez currently look a great deal like the old one that lost games at critical points of the season against less-celebrated opposition nurturing a greater hunger.
In the absence of his new £60 million striker, Wenger even turned to the departing Olivier Giroud in the closing stages of the game to try to rescue the deficit and, if this was to be the Frenchman's final game, then he will have something familiar to remember Arsenal by.
There was the dominance of possession undone by a vulnerability to the counter-attack, and a major blooper from one of the big names. That was Petr Cech, who swung and barely connected with a back-pass just after the hour and gave the ball straight to Jordan Ayew for the second goal.
Henrikh Mkhitaryan had only just come on for his debut by then and he could not retrieve the situation, along with Mesut Ozil and Giroud, and when Sam Clucas scored the second of his two goals with four minutes left, Swansea had another remarkable win.
It has been an astonishing start for Carlos Carvalhal, who has now taken 10 points from five games and beaten Liverpool and Arsenal as well as Watford, lifting his club out of the bottom three for the time being.
"When I arrived we were breathing like this," said Carvalhal, miming a wheezing patient. "When we received some oxygen against Watford, we came alive. We are breathing. We are not dead anymore. We haven't achieved anything so far, but we're in a good position now. We have a lot of work to do."
Carvalhal recently said that "20 out of 100" people might believe that Swansea could survive, so what number does he put that at now? "35 out of 100."
That figure, low though it is, would still be higher than the number of people who reckon Arsenal will make the top four after a defeat that leaves them eight points off Liverpool.
The compound it all, their best player still hasn't signed a contract that will tie him to the club beyond the summer but, again, it was Mesut Ozil who showed the quality to provide Arsenal's opening goal.
Arsenal had passed patiently before Ozil rolled the ball forward with his studs and shaped a pass to the back post and beyond Kyle Naughton, who could not stop Monreal arriving at the back post to score his fourth of the season.
Carvalhal turned away in disgust at the lapse in concentration that had fouled up a perfectly good plan. However, his players responded almost immediately with a goal that began when Ozil loitered too long in possession on the right side in his own half and was separated from the ball briskly.
Swansea did not waste time in possession and Clucas was sent on his way to beat Cech with his left foot at the goalkeeper's near post.
A poor moment for Cech but not as bad as what was to follow as, just after Mkhitaryan was introduced, the goalkeeper went to clear Shkodran Mustafi's awkward back-pass but could only direct it straight to Ayew, who kept his head to pick a spot past the Arsenal goalkeeper.
Clucas rounded off a memorable victory late on but, for Wenger and Arsenal, it will be memorable for all the wrong reasons.