Nottingham Forest boosted their fight to avoid a relegation battle and put a dent in Brighton's hopes of automatic promotion from the Sky Bet Championship with a 3-0 win at the City Ground.
Chris Hughton's side had not lost in six games on the banks of the River Trent, but suffered a second consecutive league defeat for the first time since September, as the Reds edged what was often a nervy, scruffy encounter.
Brighton were furious that Forest's first goal was allowed to stand in the 60th minute, with Britt Assombalonga attempting to get the final touch to a goal-bound shot from Zach Clough. The visitors felt Assombalonga had made contact - from a clearly offside position. But the goal was allowed to stand.
Forest made certain of three points as a blunder from David Stockdale handed Ben Osborn a second Forest goal in the 90th minute, before a Clough penalty, six minutes into injury time, added gloss to the scoreline.
Both sides were desperately in need of points, for very different reasons - and it lead to a cagey, cautious first half, in which neither team seemed willing to take too many risks.
Jordan Smith had to make a smart save to deny Anthony Knockaert, after he had cut in with purpose from the right side to send a stinging shot across the face of goal, towards the far post.
But it was the home side who were carving out the larger number of opportunities, even if none of them were anything more than half sights of goal from distance.
Jamie Ward had four efforts from the edge of the box on his own, in the form of a free-kick driven straight into the defensive wall, two shots that were deflected wide and another that arrowed high into the stands.
Assombalonga was not too far away with a dipping effort that dropped just wide, but Forest were finding it hard to turn short spells of pressure into clear openings, amid a scrappy, disjointed contest.
A battling determined run from Ward gave him the room to cross from the left early in the second half, but the ball eluded Assombalonga by a matter of inches in the centre.
A Knockaert free-kick was headed back across goal by Lewis Dunk, with Shane Duffy bravely attempting to convert at the far post, only to prod wide and smashing into the frame of goal in the process, as Brighton came close.
But it was Forest who were to take the lead, in slightly controversial fashion. When Matty Cash cut a ball back across the face of the penalty area, substitute Clough's first touch after coming on was a curling shot that was bound for the back of the net. Assombalonga clearly attempted to make a connection to guide it home, but did not appear to touch the ball.
Brighton argued that he had made contact - and that he had done so from an offside position. But the goal was allowed to stand and Forest had a precious lead to hold on to.
Seagulls sub Solly March almost made an immediate impact himself, with a fierce low shot that Smith did brilliantly to push around the post.
While Knockaert might have done better with a late header, after being picked out at the far post by the lively March.
Forest made sure of victory in the 90th minute, albeit with the aid of some serious help from Brighton, who laid a second goal on a plate for their hosts, as keeper Stockdale made a hash of things, as he dealt with a back pass - only succeeding in flicking the ball to Osborn, who slotted into an empty net.
That was not the end of the scoring, as Dunk felled Forest sub Ben Brereton to concede a 96th-minute penalty - which Clough duly converted emphatically.
Nottingham Forest boss Gary Brazil:
"It was a goal. It went into the top corner and we have had a lot of goals go into the top corner against us lately. We have been hurt a lot ourselves in games, in critical moments.
"In key moments in the game we were strong. If we have got a little bit of luck, I think we have earned it. Our all-round performance was one that deserved victory. It was not a defining moment in the game, because there was still a lot of football to play.
"In the first half we got a real grip of the game and then we pushed on from that in the second half. I am chuffed to bits for the players and the staff, but particularly for the fans who stuck with us throughout the game."
Brighton boss Chris Hughton:
"I have seen it again and the player looks offside. Whether the referee knew if Assombalonga had touched the ball or not, I am not so sure. He must have been made aware of it. But he looked offside and that goal certainly changed the course of the game.
"I was disappointed that the goal was not overturned. It looked as though he was in an offside position. The person with the best view of that is the linesman and you would expect him to give that decision.
"If he does not know if Assombalonga has touched the ball, you can understand it. But if he has been made aware of the fact that he has, that is different."