Tiemoue Bakayoko lost concentration for a fraction of a second and Saul Niguez slipped away to put a very different complexion on Chelsea’s European campaign.
As Bakayoko froze, Saul dashed to the back post, where he connected with a corner flicked on by Fernando Torres and headed Atletico Madrid into the lead.
The goal, early in the second half, could not save Atletico’s Champions League ambitions but it may have lingering repercussions for the Premier League champions.
Chelsea were able to summon a late charge and equalised when former Manchester City defender Stefan Savic deflected a low drive by Eden Hazard into his own net — and there were plenty of chances for Chelsea to win the game.
Atletico goalkeeper Jan Oblak produced a splendid save to thwart Alvaro Morata and Willian sliced over from the edge of the area when the goal gaped. Michy Batshuayi almost produced a stunning winner in the closing seconds and Hazard, inexplicably, drifted offside when he and Willian had the visitors stretched on another counter-attack.
So, Bakayoko was not the sole culprit and yet his error will loom large at the draw in Switzerland on Monday. It will also be lodged in the mind of owner Roman Abramovich, who was at Stamford Bridge on Tuesday night.
Victory would have secured Chelsea first place in Group C and the relative comfort of top seeding when the ties are pulled out for the last 16, but the draw allowed Roma — who beat Qarabag on Tuesday night — to ease ahead by virtue of their head-to-head results.
With the draw Diego Simeone’s team go into the Europa League, having started the group poorly and failed twice to beat Qarabag, the minnows from Azerbaijan.
They arrived at the Bridge in need of a minor miracle. Not only did they have to win but also hope Roma slipped up.
Neither of those things happened but their perilous situation gave the match a frantic edge as the red-and-white shirts piled forward and Simeone rolled out his full repertoire of angst.
Torres, on his first return to Chelsea where he endured a torrid time as a £50million misfit, unleashed the first shot. It was deflected over but his assist for the opening goal must have been sweet; so, too, the warm ovation he received from a generous home crowd when he was replaced immediately afterwards.
Filipe Luis was another Atletico player back on familiar turf but there was no sign of Diego Costa as he counts down the days until he can play again in January.
Left back Luis struck a post in the second half and Gabi headed the rebound into the grateful hands of Thibaut Courtois, who spent three years at Atletico.
At the other end Oblak, who is on Chelsea’s shortlist of possible replacements should they lose Courtois, was in brilliant form.
The Slovenian kept Morata at bay in the first half and made a fine save, low to his right, to deny Davide Zappacosta.
After the break, Oblak saved from Andreas Christensen and Morata again. Conte sent on attacking substitutes and Chelsea hurled men forward in a desperate bid to reclaim the initiative.
But there would be no winner and now Monday’s draw seems a little more daunting.