Oscar slipped to send a penalty over the crossbar and blow a vital Chelsea victory against Watford in interim manager Guus Hiddink's first match in charge.
Diego Costa had already scored both of Chelsea's goals, first giving his team the lead and then equalising after Watford went in front through Troy Deeney and Odion Ighalo, when they were awarded a penalty in the 78th minute.
Oscar unexpectedly wasted his effort, and in doing so allowed in-form Watford to secure a 2-2 draw, to remain in seventh place, and to leave Chelsea with much work to do from 15th.
Hiddink was overseeing the first fixture of his second reign as Chelsea's interim manager, having succeeded the recently-sacked Jose Mourinho. His first, when he replaced Luiz Felipe Scolari, went so well that many of the club's fans wanted him to stay on a permanent basis, but there is little question the circumstances this time are more difficult.
Inheriting a team just two places off the relegation zone - no matter how misleading that may seem - will have made him aware of that, and though Chelsea possess the talent to finish considerably higher, his players' confidence remains fragile.
If it had been Hiddink's plan to boost that, it rarely showed, because his playmakers were again largely reluctant to express themselves. Beyond Costa's fourth-minute header, from Pedro's cross, a goalscoring threat was minimal until they took the lead.
Watford possess one of the division's most organised defences, and as on so many occasions this season they again deserved credit for nullifying a multi-talented attack which only succeeded for the first time with their 32nd-minute goal because of reasonable fortune.
Costa, who again demonstrated a more-familiar desire with a consistently-industrious performance, showed his quality with a classy left-footed finish when John Terry had headed Willian's right-wing corner off Gary Cahill and into the striker's path.
The expectation was Chelsea would respond to scoring by playing with greater confidence but, though Oscar soon threatened by shooting over at the near-post, Watford swiftly equalised.
Inexplicably, Nemanja Matic, under little pressure, fully-stretched his arm to handball in the area, leaving referee Andre Marriner with little option to award the penalty. Deeney calmly placed his effort down the middle as Thibaut Courtois dived to the bottom left, and Watford assumed the momentum.
Their impressive start to the season has often been overlooked amid the collective struggles of Chelsea and Manchester United, but with four consecutive victories prior to Saturday's trip to Stamford Bridge they will soon be given greater attention.
At one point in the second half that run looked likely to be extended to five. Ighalo was dribbling away from goal and had already waited too long to shot when he sent the ball goalwards as his options ran out. It should have posed little threat, but instead took a heavy deflection off Cahill and went sailing in beyond Courtois' reach in the 56th minute.
Chelsea levelled nine minutes later, when Costa encouragingly produced another fine close-range finish after running onto Willian's exceptional through-ball, but victory escaped them when Oscar sent his penalty, awarded following Valon Behrami's clumsy challenge on Eden Hazard, over the bar.