Tiemoue Bakayoko switched off for a split-second, lost his man and Saul Niguez cast a very different complexion across Chelsea's European campaign.
The Niguez header, early in the second half, cost Antonio Conte's team top spot in Group C and could have lingering repercussions.
The Premier League champions managed to equalise when former Manchester City defender Stefan Savic deflected a low drive by Eden Hazard into his own net - and there were chances to win the game.
Atletico Madrid goalkeeper Jan Oblak produced a fine late save to thwart Alvaro Morata and WIllian sliced over when the goal was gaping.