Mikel Arteta celebrating Leandro Trossard's late goal. (Image: Getty)
Gary Neville immediately noticed it, as did almost everyone inside the London Stadium. Arsenal were perilously close to dropping points in their 1-0 win at West Ham after a major mistake by Mikel Arteta, a man who has rarely blinked during this title race with Manchester City.
When Ben White hit the deck after 25 minutes and needed to be substituted off, Arteta was left with a decision. The obvious one would have been to call upon Cristhian Mosquera, who has deputised at right-back for significant portions of the season, to much success, while White and Jurrien Timber picked up injuries.
Instead, he did the unfathomable and pulled Declan Rice out of midfield, shunted him to right-back, and brought Martin Zubimendi on to partner Myles Lewis-Skelly.
"He can't put Declan Rice there," Gary Neville said on commentary for Sky Sports. "I have to say that is easily prevented if you go in strong wth a real sturdy block. He swung his leg out, in a pretty weak fashion. You are asking for trouble.
"Declan Rice is going to stay at right-back, I am stunned actually. I have just been told there is no evidence Miles Lewis-Skelly has ever played right-back. But to take Declan Rice out of the middle of the pitch is a big call."
And soon enough, West Ham had their heads up and made a real contest out of a match that threatened to be dominated by the visitors.
Zubimendi and Lewis-Skelly had never played together before, Areta took a huge risk in taking Rice out of the equation in one of the biggest matches of the season, and it was completely unnecessary.
Arteta realised as much at half-time and promptly sent Mosquera out with the rest of the team, but this time Riccardo Calafiori was missing.
The Italian had created the biggest chance of the first half, but was replaced for the Ecuadorian as Lewis-Skelly moved to the left flank.
Mikel Arteta made a mess of his substitutions but turned the game back around. (Image: Getty)
It meant that Arsenal were without their first-choice left-back and now Lewis-Skelly, who excelled to such great heights in recent weeks in midfield, was pulled away from that area to accommodate Zubimendi.
It took until the hour mark for Arsenal to regain the intensity that was present when White was on the pitch, thanks to Rice's driving runs in midfield.
Zubimendi struggled to find his feet in midfield and captain Martin Odegaard was quickly prepared to come on and try and restore some dominance. Zubimendi lasted just 39 minutes on the pitch before Arteta was forced into reversing his first-half substitution completely. At least he could front up to the error - few managers would.
But on 77 minutes, with little working, David Raya came to the rescue to deny Mateus Fernandes from close range.
And then, when it mattered most, Trossard stood tall after some smart combination play between Rice and Odegaard to get the breakthrough and bedlam commenced in the away end before a late VAR check on Callum Wilson's equaliser ruled West Ham's goal out.
Arteta can thank Leandro Trossard for coming up with the goods, but the head coach's sharp U-turn on Zubimendi and Rice proved decisive.
It took a man prepared to put the team before his own pride and reputation, embodying everything this Arsenal team are all about, to put them on the brink of a first title in over 20 years.