Millwall's unbeaten run came to an end at Highbury Stadium on Tuesday night as second-half goals from Eggert Jonsson and Bobby Grant helped Fleetwood Town to a 2-1 victory.
Jonsson found the net from close range after 65 minutes to open the scoring before Grant made sure of all three points with a goal directly from a corner in the closing stages. Byron Webster's late goal ended up being a mere consolation strike for The Lions.
Neil Harris named an unchanged starting line-up as a reward to the players who had defeated Colchester United 4-1 the previous weekend, as did Fleetwood manager Steven Pressley after his side thrashed Swindon Town 5-1 last time out.
It was the hosts who started the game on the front foot, forcing Millwall back inside the opening few minutes with Jimmy Ryan's deep cross, headed clear by Shaun Cummings, their first meaningful foray into the Lions' penalty area.
Ryan's long-ranger after eight minutes was the home side's first effort on goal and just moments later they spurned a glorious chance to open the scoring when David Henen, one-on-one with Jordan Archer, was superbly denied by the Millwall keeper.
Pressley's Cod Army showed no signs of slowing up and after 18 minutes were denied what would have been a certain opener when George Saville produced a superb sliding block tackle to stop Antoni Sarcevic's effort inside the penalty area.
Heavy wind and rain around Highbury was having a significant impact on proceedings as two Millwall clearances - first from Cummings and then from Steve Morison - ended up curling back towards the Lions defence and allowing Fleetwood to keep up their pressure.
At the other end, Saville's clipped pass into the box, aimed for Gregory at the far post, was caught by the wind and floated into the arms of keeper Chris Maxwell.
Harris swapped wingers Shane Ferguson and Aiden O'Brien around after half an hour and that move almost paid instant dividends as the former intercepted a forward pass on the halfway line, charged down the right and was inches away from finding Gregory in space with a powerful left-footed pass.
The Northern Ireland international was at the heart of Millwall's next attack when Archer's long kick reached him after a flick-on by Morison, but Ferguson's low cross from the right was hacked clear by Nathan Pond.
The Lions were growing in confidence going forward and on the stroke of half-time created the game's clearest opening to that point when O'Brien's ball forward was met with a clever looping header by Morison which struck the top of the crossbar.
Henen's long-range effort, well held by Archer, was the second-half's first meaningful action. In an increasingly end-to-end affair, O'Brien then latched onto Morison's flick-on before sending a half-volley over the bar from inside the box.
Fleetwood continued to maintain their intensity going forward and David Ball, the lively forward, almost created a chance out of nothing but after twisting and turning inside Joe Martin and Mark Beevers was dispossessed and The Lions cleared the danger.
Despite The Cod Army being increasingly on top of proceedings, the visitors were just inches away from finding the net against the run of play when a long-range shot from Shaun Williams, after Gregory's lay-off, edged narrowly wide.
On 65 minutes, however, the home side were rewarded for their intense attacking play when a succession of corners ended with Tarique Fosu drilling a ball across the face of goal which was turned in by Eggert Jonsson at the far post to open the scoring.
The goal sparked Millwall back into life but Morison, charging at goal from deep, was tackled by Pond on the edge of the area as The Lions searched for an instant response.
Williams then saw another long-range effort float just wide before Fred Onyedinma, on for George Saville in the 80th minute, had a shot claimed by Maxwell at his feet.
But any chance that Millwall had of maintaining their impressive recent unbeaten run were ended after 89 minutes when Bobby Grant, on as a substitute, scored directly from a corner to the delight of the home fans.
Byron Webster did give the away side hope in the final seconds when he thumped home from close range but Fleetwood clung on for all three points.
Fleetwood Town: Maxwell; McLaughlin (Grant, 85mins), Pond, Davis, Bell; Ryan, Jonsson, Sarcevic; Fosu (Forbes, 77mins), Ball (Proctor, 83mins), Henen. Subs not used: Hunter, Haughton, Gogic, Teixeira
Scorers: Jonsson (65mins), Grant (89mins)
Millwall: Archer, Cummings, Webster, Beevers, J.Martin, O'Brien, Saville (Onyedinma, 80mins), Williams, Ferguson (Upson, 63mins), Gregory, Morison. Subs not used: Forde, Nelson, Romeo, Thompson, L.Martin.
Bookings: J.Martin, Morison
Scorer: Webster (90mins)
Att: 3,326 (away 490)