Mark Hudson suffered his first defeat as Cardiff City interim manager as his team lost at home to the Championship's bottom side Coventry City.
Victor Gyokeres scored the only goal of the game from a first-half rebound.
Callum Robinson thought he had equalised after the break but his close-range strike was controversially disallowed for offside.
Robinson then had a penalty appeal rejected and volleyed over as Cardiff searched in vain for a late leveller.
But Coventry hung on to earn only their second win of the season - and their fourth clean sheet in five games as they continue to play catch-up following their severely truncated start to the campaign.
The end of Cardiff's three-match unbeaten run under Hudson drops the Bluebirds to 16th place in the Championship table, but still only four points shy of the play-off spots.
Coventry remain at the foot of the table but, with still three games in hand over most of their rivals and judging by this performance, that position could soon change.
The Sky Blues began the day six points adrift at the bottom having mustered just one win, in which they had scored a joint-division low of eight goals in 10 games.
They had a glorious opportunity to add to that paltry total in the third minute when Cardiff goalkeeper Ryan Allsop rushed out and made a hash of his clearance.
That left the goal unattended as the ball fell to Gyokeres and, although two Cardiff defenders rushed back to try and cover their wandering keeper, the former Swansea striker still should have scored but instead curled his shot high and wide.
Jolted by that reprieve, the hosts responded with two chances for Robinson, who saw one shot blocked from Ryan Wintle's well-disguised low corner and another drift narrowly wide.
Gyokeres was then presented with an open goal he simply could not miss. Matt Godden's flick set up Jamie Allen for a shot inside the box which Allsop did well to palm away, but only to Gyokeres who tapped in from close range.
Mark Robins' Coventry were worthy of their half-time lead, but they were close to losing it five minutes into the second half.
Cardiff thought they had equalised when Robinson finished neatly inside the six-yard box but, after consulting with his assistant, referee Bobby Madley disallowed the goal after judging that Joe Ralls had headed the ball back into the penalty area for Robinson, who was offside.
Bluebirds players and supporters furiously contested the decision, insisting that it was a Coventry player who had headed the ball back into his own area.
They were angered again when Madley ignored Robinson's claims for a penalty after he fell under a challenge from Kyle McFadzean, but those appeals were optimistic at best.
Robinson only had himself to blame when he blazed over from eight yards out and, as Madley blew the final whistle, the only thing louder than the boos from the home crowd were the elated roars of the noisy travelling supporters from Coventry as they savoured three well-earned points.
Cardiff City: Allsop, Campbell (Simpson 64’), Ng, Kipré, Nkounkou (Ojo 71’), Wintle, Ralls ©, Rinomhota (Sawyers 64’), O’Dowda (Philogene 81’), Harris (Etete 81’), Robinson. Subs not used: Alnwick, Nelson.
Coventry City: Wilson, Panzo, Doyle (Rose 23’), McFadzean ©, Allen (Palmer 75’), Sheaf, Gyökeres, Dabo, Godden (Waghorn 75’), Bidwell, Eccles. Subs not used: Moore, Kane, Tavares, Burroughs.