Arsenal boss Arsene Wenger knew his side were on a hiding to nothing after five minutes against West Brom.
Albion inflicted the first home defeat on the Gunners since Manchester United last January as Wenger's side failed to close the gap on leaders Chelsea at the top of the Barclays Premier League.
Albion, who enjoyed their first victory at Arsenal for 27 years, were 3-0 in front in the second half thanks to goals from Peter Odemwingie, Gonzalo Jara and former Gunner Jerome Thomas.
Samir Nasri claimed two late goals but Arsenal could not find the key to gain anything out of the game and the Baggies held on to win 3-2.
Wenger is hoping that his squad can put the defeat behind them in time for their game against Partizan Belgrade in the Champions League on Tuesday night.
"After the first five minutes it was all flat," said Wenger.
"It is unusual for us. We are a team who has energy. It was not there.
"What is most frustrating was there was no electricity. We had lost the generator. And then it becomes difficult to play football.
"It is mysterious what happened. It is difficult to explain. We will never forget it but we play on Tuesday night. So, of course, we have to analyse what happened. This is very difficult to understand."
But Wenger refused to blame goalkeeper Manuel Almunia who allowed Jara's shot to bounce off his arms and into the net in the 52nd minute.
His error came after his first half penalty save that saw him keep out Chris Brunt's effort.
"What happened today people will say was because of Almunia," added Wenger.
"But it was 0-0 at half-time because of Almunia. So, I believe, individually, defensively, everyone made mistakes."
West Brom boss Roberto Di Matteo has taken to enjoying the odd cigarette or two after winning in the Premier League as he bid to cleanse the club of its yo-yo promotion and relegation image.
"Our pressure this season is to get rid of the yo-yo reputation of the club," said Di Matteo.
"We got promoted last year which was a hell of an achievement but the fact this result was against Arsenal probably ranks high up there in terms of results.
"Usually I smoke one cigarette after a game but after Arsenal it was two. It just gives me four or five minutes to collect my thoughts. Obviously I do not smoke in the stadium as it is not allowed."
Arsenal were largely undone by Thomas' contribution against his old club. Thomas, sent off in an altercation with Jack Wilshere when the two clubs met in the Carling Cup last year, was a constant thorn in Arsenal's side.
But there was no bitterness from the winger who left Arsenal in 2004 to pursue life elsewhere - even from last year's red card.
"It was the first red card of my career as well, said Thomas. "It was disappointing but I had to put it behind me.
"There is never really a point to prove, you just want to do well in every game. But it is an added incentive when you come back to your old club.
"When I was there Robert Pires, Patrick Vieira, Dennis Bergkamp and Thierry Henry were in front of me. Now the standard, no disrespect to the Arsenal team, is not as high.
"I chose to leave. I was offered another year's contract, I wasn't released. There was no regrets, I don't hold any bad feeling.
"You can't put a price on the education I got at Arsenal which is why you can never be bitter when it's time to move on."