Hooker Rob Rawlinson's yellow card started the rot for Leeds at Castle Park
Leeds Carnegie surrendered their 100% record as they succumbed to their first National One defeat of the season at local rivals Doncaster.
A try in each half from Dan Smith and Bevon Armitage were the centrepiece of the Knights' surprise victory.
But home fly-half Jamie Lennard played his part, supplying over half the points thanks to three penalties, a drop-goal and one conversion.
Hudson Tonga'uiha also dropped a goal as much-changed Leeds were stunned.
The turning point of the game came on 33 minutes when, with the score still 0-0, visiting hooker Rob Rawlinson was sin-binned for a fourth penalty infringement inside Leeds' own 22.
Lennard, who had already missed with two penalty attempts, slotted home this one. And 14-man Leeds were made to further pay before the break.
Just four minutes later, Smith finished off a flowing move in the left corner. And, although Lennard failed with the kick, Tonga'uiha stepped up the plate in stoppage time to add the drop-goal that stretched his side's lead to 11-0.
Inside eight second-half minutes, Lennard added a drop-goal of his own before two more penalties. And, when Armitage won the race to collect a charged-down drop-goal attempt, Lennard added the extras to make it 27-0 with just 13 minutes left.
Leeds, who pulled off a miraculous escape to stay in the National Trophy with two late tries at Birmingham and Solihull a week earlier, were not finished yet.
Two tries in eight minutes out wide by Leigh Hinton and Richard Welding, the second of them converted by Alberto Di Bernardo, offered slim hope.
But this time it was too little, too late as Doncaster, who climb a place to third, hung on to record a famous victory.
Leeds now lead by just eight points from Exeter, who ran in five tries in beating Manchester 40-5 and still have a game in hand.