Two goals in four frenetic minutes by Lee Frecklington and Paul Taylor left The Lions wondering what might have been as Kenny Jackett's side surrendered a 2-0 lead to let Peterborough United secure a share of the spoils in a pulsating Den encounter.
There was plenty of thrills and spills in this four-goal feast, yet for the second time this season Millwall let their grip on a seemingly match-winning position slip.
It was a night of mixed emotions for Lions boss Jackett, who had some positives to take from this performance - namely midfielders Liam Trotter and Hamer Bouazza getting on the scoresheet, the oustanding contribution of Jimmy Abdou in the engineroom and the immense display of skipper Paul Robinson at the back.
But the manner in which The Lions took their foot off the gas and subsequently let a 2-0 lead slip will be of some concern and immense frustration.
It's fair to say that Posh owed a huge debt of gratitude to keeper Paul Jones to even be in a position to claw their way back into the game, with the former Exeter City stopper making a string of top drawer saves.
However, that should not take away from the fact that The Lions relaxed, enabling Darren Ferguson's side to haul their way back to level terms and secure what had seemed the most unlikely of points.
Jackett made a tactical switch to try and negate the attacking threat of Posh, bringing Jimmy Abdou into the team in a compact midfield alongside Trotter and Tam Mkandawire, with John Marquis dropping to the bench.
Darius Henderson was the focal point of a three-pronged Lions attack, flanked by the ever-willing James Henry and Bouazza, with the duo's pace and flair posing a consistent and effective threat to the visiting defence.
It took a while for The Lions to settle into their new formation and Posh created the first real chance after five minutes when midfielder Tommy Rowe fired narrowly over from around 14 yards.
Henry then saw his stinging 25-yard free-kick clawed away by keeper Jones, before Lions skipper Robinson produced an excellent block to deny striker David Ball from close range as he prepared to shoot.
Abdou's looping header from a deep Tony Craig left-wing cross dropped wide of the post and then George Boyd smacked a shot against the foot of the post after home keeper David Forde had fumbled Grant McCann's 17th-minute corner.
The two teams continued to trade scoring chances as Forde was then forced to turn Grant McCann's 30-yard drive round the post, before Lions striker Henderson saw a flicked header pushed away yet again by Jones at the other end.
Jones was called into action twice in as many minutes, thwarting Bouazza firstly from a tight angle and then diving low to his right to push another daisy-cutter from the Algerian international round the post.
Henry was denied by the Posh stopper after half an hour before Millwall finally found a way through on 32 minutes, Trotter sweeping home Henderson's cross from close range for his second goal in successive games.
Trotter went close to grabbing his - and Millwall's - second goal just before the break, latching onto Henderson's flick and volleying goalwards, only for that man Jones to athletically tip his effort away.
The Lions did grab their second goal though, and what a corker it was too, 10 minutes after the restart. Henderson was again the provider, slipping a delightfully weighted ball through to the former Watford man. He in turn shimmied his way into the area, before arrowing the ball into the bottom left corner with a clincial finish.
It looked to be game over at that stage with The Lions having the measure of Posh. Jackett's men were, however, in for a rude awakening.
Posh grabbed a goal out of nowhere with a superb finish from Frecklington, who flicked up Ryan Tunnicliffe's 72nd-minute pass and volleyed past Forde to halve the deficit.
The Lions could have restored their two-goal advantage three minutes later when Jones again came to the rescue, this time tipping Trotter's curling shot onto the post.
That added further belief in the visiting camp that they could, indeed, get something out of this game.
And just 60 seconds later that proved to be the case. Skipper Robinson appeared to be fouled 10 yards inside his own half, but play was waved on and the ball found its way to substitute Taylor who had time and space to fire a low drive beyond the desperate reach of keeper Forde - to the despair of The Den faithful.
Trotter had a golden opportunity to tip the balance back Millwall's way when he picked up Bouazza's deflected shot, but failed to connect cleanly and Jones gathered with ease.
The introduction of John Marquis and Josh McQuoid coupled with five minutes of injury-time failed to conjure up a third, match-winning goal for the home side, who have no time to dwell on what might have been with a trip to league leaders Southampton looming on the horizon this Saturday.
Millwall: Forde, Dunne, Craig, Trotter, Robinson, Ward, Bouazza, Mkandawire, Henderson (Marquis, 83mins), Abdou (McQuoid, 83mins), Henry. Subs not used: Mildenhall, Smith, Hackett.
Booking: Dunne (73mins)
Scorers: Trotter (32mins), Bouazza (5mins)
Peterborough United: Jones, Little, Bennett, Wootton, Tomlin, Ball (Taylor, 55mins), Boyd, McCann, Rowe (Tunnicliffe, 65mins), Frecklington (Basey, 88mins), Alcock. Subs not used: Day, Thompson.
Bookings: Tomlin (45+1min), Taylor (82mins)
Scorers: Frecklington (72mins), Taylor (76mins)
Att: 10,240 (away 443)