Millwall suffered a first defeat in eight matches on Saturday afternoon as a 94th-minute Barnsley goal saw The Tykes win 2-1 at The Den in the Sky Bet Championship.
After completely dominating the opening 40 minutes of the contest, The Lions fell behind to Conor Chaplin's strike, but Aiden O'Brien stepped off the bench to net the equaliser with just five minutes left.
Millwall were dealt a cruel, cruel blow four minutes into stoppage-time, however, as substitute Patrick Schmidt's header won the game for the visitors.
Millwall boss Gary Rowett made two changes to his side for the visit of The Tykes, both through illness, as Murray Wallace and Alex Pearce missed out and were replaced by Shane Ferguson - returning from injury - and Jiri Skalak.
Jason McCarthy, meanwhile, also returned from injury to take a place on the bench.
Both sides struggled to retain possession in the opening moments, but with six minutes on the clock, Jake Cooper's fantastic, flat cross-field pass found Mahlon Romeo - he slipped Jed Wallace into the penalty area, but the winger's cross was blocked behind, with the resulting corner being cleared. That move gathered a bit of momentum, with Cooper very close to getting his toe to the ball a minute later, but once more Barnsley cleared to safety.
Jed Wallace's corner to the back post was then headed away as Millwall started to gain the upper hand against their lowly opponents, before Jayson Molumby tumbled inside the area on 13 minutes - referee Jarred Gillett, however, remained uninterested.
The Lions broke away with 16 minutes on the clock with Jon Dadi Bodvarsson feeding Skalak - he then laid the ball off for Ferguson to swing a cross in, but former Barnsley man Tom Bradshaw's header went the wrong side of the near post.
The Tykes were giving away an incredible amount of fouls in the first 20 minutes, and another Millwall set-piece was swung over by Jed Wallace, however this time it was the head of Cooper getting to the ball, who glanced wide. The same pair then combined from yet another free-kick three minutes later, but again the ball went wide.
The corners, free-kicks and regaining of possession kept on coming as the half-hour mark came and went, but still The Lions could not unlock the Barnsley door. At the other end, the visitors finally made their first foray into the penalty area on 38 minutes - and just 60 seconds later, took the lead completely against the run of play as Chaplin curled a shot past the onlooking Bartosz Bialkowski, who had barely touched the ball all game.
From nowhere, Barnsley were now in the ascendancy and if not for Bialkowski, Chaplin would have doubled his tally and the visitors' lead in first-half stoppage-time - the Pole pulling off a great save to deny the attacker.
Early in the second-half The Tykes found a way to goal again, but once more Bialkowski was to the rescue, getting down low to save from Jacob Brown. Alex Mowatt then made the stopper work once more, before the captain's follow-up effort hit the roof of the away stand behind the goal.
Jed Wallace then battled his way into the penalty area but found his shot blocked as the Den crowd looked to raise the volume inside the stadium, and with a Ferguson corner knocked to the back post, Cooper couldn't find the net. The number seven then fizzed a cross across the box, but Bradshaw's snapshot went over the bar.
Matt Smith and Aiden O'Brien were introduced on the hour-mark as Gary Rowett looked to reshuffle his pack, and after Bambo Diaby's up and under allowed the duo and Jed Wallace in, the latter's shot was blocked. Smith then punished Barnsley goalkeper Samuel Radlinger and caught him in possession outside the box, but the substitute's effort was tame.
Two Irishman combined with a little over 10 minutes to go as Shaun Williams crossed for O'Brien, but Aiden's looping effort dropped the wrong side of Radlinger's net. The sub didn't have to wait long for another chance though, as a cross was nodded up in the air by Smith before being mopped up by O'Brien, who found a route to goal for the equaliser.
The Tykes lost their discipline in the immediate aftermath of the strike, with first Diaby booked for flooring Williams and then Aapo Halme somehow escaping more punishment than just a yellow card for deliberate handball and a shove on Molumby.
There was a late chance for Millwall to win the game, but as Cooper rose highest from Jed Wallace's free-kick, the defender powered a header over the crossbar. Barnsley then broke away down the other end and Mowatt unleashed a strike that looked destined for the net, but Bialkowski was there to stop.
The action wasn't over there, though, as Schmidt glanced past the goalkeeper four minutes into stoppage-time to win the game.
Millwall: Bialkowski, Hutchinson, Cooper, S. Williams, J. Wallace, Bradshaw (Mahoney 72'), Ferguson, Romeo, Molumby, Bodvarsson (Smith 60'), Skalak (O'Brien 60'). Subs not used: Steele, McCarthy, Brown, Mitchell.
Scorer: O'Brien 85'
Booking: J. Wallace
Barnsley: Radlinger, J. Williams, Diaby, Andersen, Brown, Woodrow (Schmidt 84'), Bahre (Thomas 72'), Chaplin, Oduor, Halme, Mowatt. Subs not used: Collins, B. Williams, Styles, Sibbick.
Scorer: Chaplin 39', Schmidt 90+4'
Booking: Andersen, Woodrow, Diaby, Halme
Referee: Jarred Gillett
Attendance: 12,682 (614)