Ben Davies' goal in first-half stoppage-time condemned Millwall to defeat at Fleetwood Town on Easter Monday.
Despite 16 corners and 15 shots on goal, The Lions were not able to find a way past an inspired Alex Cairns in the Fleetwood goal.
Millwall boss Neil Harris named an unchanged starting XI from Good Friday's 3-0 demolition of Northampton Town, with just one change on the substitutes' bench, as Shane Ferguson replaced Jimmy Abdou.
Five minutes in, The Lions had the first opportunity to take the lead, but Ben Thompson's shot found the post. From the rebound, Lee Gregory's shot was expertly saved by Cairns.
Millwall started the brighter and had a further attempt from an acute angle on six minutes, but Steve Morison's effort was palmed away by Cairns.
Fleetwood's first shot on goal, meanwhile, came on 13 minutes from Bobby Grant, but he shot high and wide. Shaun Hutchinson headed Shaun Williams' corner-kick on to the roof of the net two minutes later as The Lions looked to take the lead.
The Lions' chances kept coming, but this time, on 17 minutes, Morison's half-volley from another Williams corner-kick was saved. In what was sheer one-way traffic, the away side were to hit the woodwork once more on 22 minutes, as Gregory guided an effort on to the post following good work by Jed Wallace.
Grant forced Tom King into his first save of the afternoon from a whipped free-kick on the half-hour mark, whilst indecision in The Lions' defence caused Byron Webster to head over from underneath his own crossbar.
A minute into first-half stoppage-time, Fleetwood took the lead. A free-kick found its way deep into the Millwall penalty area and on to the head of Ben Davies, who found a way past King to open the scoring.
The second-half was 10 minutes old when Millwall had an opportunity to draw level, but after a bout of pinball in the box, Gregory's goal-bound shot was blocked by a Fleetwood leg.
Gregory's neat flick-on found the on-running Morison on 63 minutes, but Cairns ran out to smother the ball just as the striker shaped to shoot.
Two minutes later, substitute Jake Cooper headed Williams' corner-kick towards goal, but Cairns extended brilliantly to make a superb save.
The almost absurd succession of corner-kicks continued for The Lions, but chances from O'Brien and Webster went begging as Millwall tried to find a way back into the game.
The barrage continued, but Cairns stood firm once more with 84 minutes on the clock, as the 'keeper turned Gregory's snapshot away for a corner-kick.
In the final seconds of stoppage-time, substitute Calum Butcher headed towards goal, but Cairns was on hand once more to keep The Lions at bay.
Fleetwood Town: Cairns, McLaughlin, Bell (Nirennold, 56'), Brannagan (Schwabl, 87'), Eastham, Pond, Burns (Dempsey, 61'), Ball, Grant, Glendon, Davies. Subs not used: Urwin, Davis, Hunter, Woolford.
Bookings: Nirennold, Glendon
Scorer: Davies (45+2')
Millwall: King, Martin (Ferguson, 77'), Hutchinson (Cooper, 46'), Williams, Thompson (Butcher, 82'), Gregory, Romeo, Wallace, Webster, Morison, O'Brien. Subs not used: Archer, Cummings, Worrall, Onyedinma.
Referee: J Brooks
Attendance: 3,655 (656)