Neil Warnock returned to haunt Sheffield Wednesday as his Rotherham United side ended the Owls' remarkable home record on a scrappy and controversial afternoon at Hillsborough.
Matt Derbyshire gave the Millers a 21st minute lead which they would not relinquish, giving their Championship survival hopes a real shot in the arm.
And Wednesday could be left counting the cost of what was only their second home defeat of the campaign and first since August.
Defeat - coupled with a victory for Cardiff City earlier in the day - means the Owls' cushion inside the top six has been eroded completely, with only goal difference leaving them inside the play-off positions.
The hosts struggled greatly in the first half and deservedly went behind. Bizarre officiating saw an equaliser from Aiden McGeady ruled out for offside, albeit correctly.
And while Wednesday were much improved in the second half, they struggled to break down a Rotherham side that worked its collective socks off.
Grant Ward had a glorious chance to give the Millers the lead just two minutes. in. He watched Derbyshire's lofted ball over his shoulder and volleyed, seeing his shot curl just wide.
And the quick start for the visitors would continue as they consistently punished sluggish play from Wednesday.
The Millers, to their credit, pressed the hosts superbly, forcing plenty of errors in the middle of the park. They also got players back behind the ball incredibly quickly, presenting Wednesday with an imposing yellow wall whenever they were in possession.
So when Rotherham took the lead on 21 minutes, it was more than deserved for their work rate alone.
Lee Frecklington sent a low ball across the face of goal which no Wednesday defender got near, allowing Derbyshire to turn in at the back post.
Wednesday struggled to threaten their struggling visitors until a moment of true controversy when they thought they had equalised.
A combination of McGeady and Michael Turner looked to have drawn Wednesday level only for a bizarre sequence of officiating which led to it being chalked off.
McGeady carried the ball from the byline and drilled on goal from 15 yards. Turner unsighted goalkeeper Lee Camp and the shot nestled in the back of the net.
As Wednesday celebrated, Rotherham's players surrouned the assistant referee on the far side. After a few moments of discussion and contemplation, he raised his flag to signal for a free kick.
Referee Christopher Kavanagh consulted with his assistant and then pointed for the free kick himself, much to the ire of the home support at Hillsborough.
While the decision was correct, it is the manner in which it was made that will see it debated for the days to come.
The Millers really should have been 2-0 up moments later. Ward darted in to meet a zipping cross unmarked and volleyed just wide of the near post.
Wednesday enjoyed their most threatening period in the later stages of the half. And they were inches away from an actual equaliser when Jack Hunt drilled narrowly wide from the edge of the box.
Carlos Carvalhal threw on Lucas Joao for the ineffective McGeady at the break and the Portuguese striker made a big difference for the Owls.
Joao added energy and directness to Wednesday's play and they began to force the Millers deeper, putting them under pressure.
Wallace was the width of the bar away from drawing the hosts level. He collected a high ball, lined up a shot and curled an effort back off the woodwork.
Wednesday again thought they were level on the hour mark. Joao smashed a shot from 35 yards, Camp parried and Gary Hooper turned in the rebound only for to be flagged for offside.
Wednesday pushed further. A beautiful turn 30 yards out from Joao put him in clear sight of goal. Though his touch was heavy, he managed to get a shot away which Camp tipped wide for a corner.
Wallace forced a saved from Camp from a free kick just outside the area, the Millers keeper stopping the curling effort with his legs.
Try as they might, Wednesday could not break the Millers down. And it was the visitors who went closest to grabbing the game's second goal when Richie Smallwood clipped a shot wide after being put one on one with Keiren Westwood.
Wednesday: Westwood, Hunt (McGugan 74), Lees, Turner, Pudil, Wallace, Lee, Bannan, McGeady (Joao 46), Hooper, Nuhiu. Subs not used: Price, Loovens, Bennett, Lopez, McGugan, Matias.
Rotherham: Camp, Richardson, Broadfoot, Wood, Mattock, Halford, G Ward (Kelly 90), Frecklington, Smallwood, D Ward (Best 90), Derbyshire (Newell 82). Subs not used: Kenny, Thoas, Green, Clarke-Harris.
Referee: Christopher Kavanagh (Lancashire)