Mikel Arteta was unhappy with Arsenal's set-piece defending after Bayer Leverkusen used their own weapon against them in their 1-1 draw in the Champions League last 16 at the BayArena.
Arsenal's set-piece threat had been a topic of discussion ahead of the first leg but Leverkusen showed some of their own when Robert Andrich headed in Alejandro Grimaldo's inswinging corner.
The goal, which was later cancelled out by Kai Havertz's penalty against his former club, came just 45 seconds after half-time and followed Arsenal nearly being caught out from a kick-off routine.
David Raya's superb save from Martin Terrier's header kept the scores level from that chance, but Leverkusen punished slack marking to score from the resulting set-piece.
The goal was hugely frustrating to Arteta, who said he and his coaching staff had shown their players three examples of Bayer Leverkusen's kick-off routines from their previous game.
"We discussed that at half-time and we expected it to start very fast, especially because we knew certain routines that they have on kick-off, because they had three on the weekend," he said.
"Instead we get caught, we're not ready enough, we concede a chance on the header, and that's when we concede the goal.
"When you give that hope and advantage to the opponent, at that level, they're going to take it, and then we had to really dig in because emotionally it becomes a very different challenge."
The goal was the first Arsenal have conceded from a set-piece in this season's Champions League and Arteta felt it resulted from complacency from his players.
"There's always two sides to that," added Arteta. "One is the element of the opponent, that they picked that weakness and that lack of attention or urgency in both situations.
"And the other one is us because we knew, we showed them three clips from last weekend in three different ways but we weren't ready for that and we got caught."
Jover to Hjulmand: 'You do it too, eh?'
After Andrich's goal, Bayer Leverkusen head coach Kasper Hjulmand was seen having a brief exchange with Arsenal's set-piece coach Nicolas Jover on the touchline.
Hjulmand had been critical of the blocking tactics used by Arsenal on set-pieces at his pre-match press conference but insisted after the game that his comments were misinterpreted.
The Dane said he had intended to question the rules, rather than Arsenal specifically, and that Leverkusen applied the same tactics, something he revealed he admitted to Jover after their goal.
"I think there was a little bit of a misunderstanding from my press conference yesterday that I was challenging the way Arsenal are doing things with the offensive blocks," he said.
"It's not about Arsenal, it's about the rules. We do the same. All teams do. We put offensive blocks, we body-check to make space, and I'm just questioning, is that legal? Is it actually in the rules that you can take players out without the ball?
"So he just said, 'you do it too, eh?'
"'Yeah, we do it too!'
"It's the same for all teams. We are all doing it."
Hjulmand took issue with the award of the penalty converted by Havertz, when the referee's decision to penalise Malik Tillman for a foul on Noni Madueke was upheld by VAR.
"For me it's not a penalty," he added
"I think that sometimes the referee shouldn't blow the whistle, and then check with the VAR, then it's 100 per cent safe and secure. But if he whistles, then it's not clear and obvious. For me it just wasn't.
"It would have been better for us with a 1-0 victory but that's how it is. I don't see a penalty there but that's the way it went."
Penalty a 'big moment' for Havertz
The decision frustrated Leverkusen but allowed Havertz to play a decisive role against his boyhood club, who he left to join Chelsea six years ago, by scoring the third goal of an injury-hit season.
"Football is a funny game and it brings special stories," said Arteta.
"Him coming back here after such a long time, being part of this club, and scoring such an important goal, I think it's a big moment."
Havertz admitted the long wait to take the spot kick, as the decision was checked and the referee addressed encroaching, made it tougher but said he was helped by the familiarity of his surroundings.
"The time between the penalty whistle and the penalty kick felt like an eternity, of course," he told UEFA.
"But in the end, you have to be mentally present in those moments.
"I know the stadium well and have taken many penalties from this spot. That's why I felt good.
"Another big task awaits us next week."