Yanic Wildschut endured an evening to forget as his defensive error contributed to Cardiff suffering a damaging defeat at Derby.
The on-loan Norwich City winger came on in the 74th minute shortly after former Canaries striker Cameron Jerome had shown good strength to equalise for the play-off chasing hosts.
Wildschut, making his 10th appearance since joining the Welsh club in January, got himself in a muddle in possession in his own penalty area eight minutes after coming on and was robbed by former Norwich loanee Andre Wisdom. The right-back crossed and the Championship's top scorer, Matej Vydra, rifled the Rams in front.
The Dutchman slumped to the turf with his head in his hands knowing that he was at fault and then also couldn't keep possession in midfield ahead of Jerome muscling his way to another goal late on to seal a vital 3-1 win for Derby.
The result moved County back up to sixth with two games left, making the most of their game in hand on their rivals, and leaves Cardiff a point clear of Fulham in second place.
Elsewhere on Tuesday night, Ben Godfrey and Carlton Morris saw their automatic promotion hopes come to an end in League One despite Shrewsbury winning 3-1 at home against Peterborough.
Blackburn joined Wigan in celebrating automatic promotion thanks to a 1-0 win at Doncaster, as Godfrey played the opening hour of the Shrews' win but Morris remained on the bench, which guarantees them third place with two games remaining.
They could face Plymouth and Remi Matthews in the play-offs but Argyle missed a big opportunity to move level on points with sixth-placed Scunthorpe as they were held to a 1-1 draw at lowly Rochdale.
City keeper Matthews made two good stops to deny the hosts a winner in the second half, leaving the Pilgrims seventh but firmly in the play-off mix, three points behind fifth-placed Charlton and with a game in hand.
MORE: City loanee focuses on Plymouth play-off chase with wife and daughter on the mend
Ebou Adams also played the full match as Leyton Orient were beaten 1-0 at home by Maidenhead in the National League.
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