Goals from Gary McSheffrey and substitute Jim O'Brien condemned Millwall to a first League defeat in seven as Scunthorpe United were left celebrating a 2-0 success at The Den on Tuesday night.
McSheffrey found the back of the net after 17 minutes, making the most of a defensive error, before O'Brien made sure of a fourth straight victory for The Iron with a goal deep into stoppage time.
Neil Harris recalled Shane Ferguson and Aiden O'Brien to the starting line-up in place of Joe Martin and the suspended Shaun Williams, and it was his side who carved out the game's first opening when Byron Webster's header made its way to Lee Gregory, who couldn't quite turn the ball home from close range after four minutes.
Despite a shaky start in defence, the visitors grew into proceedings early on and should have gone in front when Paddy Madden saw one cross blocked before a second reached Tom Hopper, who controlled, turned and fired wide of Jordan Archer's left-hand post from close range.
That seemed to spark Millwall back into life and a smart move down the right almost resulted into an opening goal but Jed Wallace's cross after a cut-back from O'Brien was turned into the side netting by Gregory.
Just seconds later, though, The Lions conceded for the first time in just under 300 minutes of football. Sid Nelson did well to win the ball back midway in his side's defensive half but then conceded possession to Neal Bishop, who instigated a slick passing move which ended with McSheffrey firing past Archer and into the far corner.
The home side's play was a little disjointed but they almost equalised 10 minutes later when Wallace's dangerous cross just evaded both Gregory and Steve Morison, who had made a run to the far post.
That opportunity marked the start of a steady improvement to Millwall's play, and two long-range efforts from O'Brien and Wallace were further indications of their attacking intent.
There was an additional blow shortly before the half-time whistle, however, as Ed Upson was forced off with an injury; Ben Thompson coming on in his place.
That enforced substitution did little to disrupt Millwall's gradual improvement in the game and their first meaningful opportunity of the second-half, after 50 minutes, saw Wallace's cross make it all the way to O'Brien, whose shot at the far post was turned behind by keeper Luke Daniels.
Thompson was beginning to make an impact on proceedings with a smart range of passing and his ball to Ferguson started a quick attack, but the Millwall left-back could only run the ball out of play after doing well to charge past Bishop.
An inventive piece of play from Wallace almost ended with an equaliser for The Lions on the hour mark but Gregory, after latching onto the loanee's pinpoint through ball, could only dink an effort narrowly wide of the post having already beaten Daniels.
With the hosts more and more urgent in their attacking play, Scunthorpe were beginning to have some joy on the counter attack but Nelson did well to hold up Madden during one swift burst into Millwall's half.
Naturally The Iron began to sit deeper and deeper, and they were almost made to pay when Ferguson's free-kick made its way to the far post for O'Brien, whose header back across goal was turned behind by Bishop in a desperate, last-ditch block.
With greater numbers in midfield after their two substitutions, Scunthorpe were able to enjoy sustained spells of possession as they looked to close the game out, though they could have made it 2-0 when Stephen Dawson's effort from inside the box was beaten away by Archer.
At the other end, Thompson's strong tackle on the edge of the box creating a shooting opportunity for Byron Webster, whose first-time effort was well saved by Daniels.
That was to be Millwall's final opportunity, though, and Scunthorpe made sure of all three points when O'Brien found the bottom corner after the home side failed to clear their lines.
: Archer, Romeo, Nelson, Webster, Ferguson, Wallace, Abdou (Marquis, 81'), Upson (Thompson, 45'), O'Brien, Gregory, Morison
Subs not used: Forde, Martin, Edwards, Beevers, Onyedinma
Scunthorpe: Daniels, Mirfin, Madden (Luer, 79'), Bishop, Dawson, Townsend, Hopper (Wotton, 90'), Wallace, Ness, McSheffrey (O'Brien, 66'), Clarke
Subs not used: Anyon, Laird, Adelakun, King
Goalscorers: McSheffrey 17', O'Brien 90+2'
Referee: Nicholas Kinseley