Arsenal manager Arsene Wenger slammed his side's defending after watching Anderlecht claw their way back from three goals down to earn a 3-3 draw in Champions League Group D on Tuesday.
The hosts were 3-0 ahead inside the hour and apparently cruising into the last 16 of the competition for a 15th successive season under the Frenchman when the wheels came off in spectacular fashion at The Emirates.
Instead of shutting up shop, Wenger's side capitulated in worrying fashion, allowing the Belgian champions to hit back with two goals from Anthony Vanden Borre, the second a penalty conceded by Nacho Monreal, before Aleksandar Mitrovic's 90th minute header left the home crowd stunned.
"Annoyed? Yes. But you have to give them credit. They did fight until the end and we gave a poor defensive performance from the first to the last today. We never felt comfortable," a rather nonplussed Wenger told reporters.
"We got punished. A bit of bad luck because the first goal was offside but our defensive performance was not good enough."
Even before Mikel Arteta gave the hosts a 25th minute lead from the penalty spot, Anderlecht had caused several anxious moments with their quick, incisive football.
However, when Arsenal's standout performer Alexis Sanchez smacked in a second and Alex Oxlade-Chamberlain made it 3-0 in the 58th minute, the home side's clear defensive frailties appeared a mere blemish to a decent night's work.
Vanden Borre was apparently offside when he pulled a goal back from close range but what happened after that left Wenger angry and perplexed.
"At 3-0 we thought the job was done. It was fatigue as well, they finished stronger and sharper. We got punished. It's very disappointing," he said.
"In the Champions League you need to be at the right mental level or you get punished. Certainly we were not. What I say in the dressing room stays in the dressing room."
Second-placed Arsenal are still in a strong position to qualify for the last 16, having seven points with two games to play, five more than third-placed Anderlecht who they beat 2-1 rather fortuitously a fortnight ago in Brussels.
"Overall we got four points from the two games. We are well paid for the two performances," Wenger said.
Former Arsenal midfielder Paul Merson, working as a Sky Sports pundit, was not being hoodwinked though.
"If I'm being honest, I think they're tactically clueless," he said. "How you can be 3-0 up and attacking. This is not the first time, this happens a lot of times.
"You're playing with international footballers. These are not little kids."
For all of Arsenal's defensive deficiencies - meaning they will almost certainly finish runners-up to Borussia Dortmund, who have 12 points, and face a daunting last-16 tie - Anderlecht's display should not be ignored.
But for a late meltdown at home against the London side they would have been right in the mix to qualify.
"I'm a young coach and I hope I will be here for a long time but this will stay in my memory," Anderlecht manager Besnik Hasi told reporters. "Maybe (Arsenal) thought that they were superior at that moment and they could have fun with the public.
"But I'm very very proud of my team and they showed today their character and their quality. They did fantastic today (Tuesday)."
Asked if Vanden Borre's first goal should have been flagged offside, he added: "It wasn't lucky, I don't care!"