Conor Thomas's last-gasp winner for Swindon Town ended Millwall's unbeaten run in League One.
With a place in the League One play-off places on offer - albeit potentially on temporarily - with a draw or better, the Lions watched as Thomas fired home in the fifth minute of added time to steal the win.
Nicky Ajose picked out the substitute in the penalty area to boost the Robins' relegation fears as they move to within four points of safety ahead of the 3pm kick-offs.
For the Lions, it was a first defeat in the league since a 3-0 reverse away at Scunthorpe on December 17.
In a game of few clear-cut chances, the first half was an even affair. Lee Gregory scouted out the first chance of note when running onto Jordan Archer's put up field but could failed to trouble Lawrence Vigouroux.
Archer was again involved soon after in what proved to be the most eye-catch moment of the game until Thomas's late heroics, clawing away a Charlie Colkett effort destined for the top corner.
Swindon were good value for the three points, enjoying most of the ball throughout, but were guilty of missing chances until midfielder Thomas snatched his first league strike of the season.