Tristan Crama struck the winning goal in the seventh minute of stoppage time as Millwall moved up to third in the Sky Bet Championship following a breathless 3-2 victory over Southampton at The Den.
Caleb Taylor's screamer had put the Lions 2-1 in front but that was cancelled out by Finn Azaz's scrambled equaliser in what was a brilliant spectacle.
French defender Crama had the final word to inflict a first defeat under caretaker boss Tonda Eckert on the Saints and move the Lions to within two points of second-placed Middlesbrough.
Aidomo Emakhu came close to an early opener for Millwall when he raced onto a loose ball before placing a low shot from just outside the area narrowly wide.
Southampton - fast starters during their recent winning run - then came close themselves in the 10th minute when Jay Robinson drove into the box before forcing a save from goalkeeper Max Crocombe.
The Lions threatened once more when Daniel Kelly's shot was diverted by Mihailo Ivanovic, with Southampton goalkeeper Gavin Bazunu getting down to his left to make an excellent save.
Crocombe then had to come to the hosts' rescue as he came out to deny Robinson for a second time after the youngster had been played in by Azaz.
The New Zealand international had to be on his toes again just before half-time when he got down well to push Adam Armstrong's effort around a post.
The breakthrough finally came in the 55th minute when Southampton were awarded a penalty for Taylor Harwood-Bellis being bundled over by Jake Cooper from Ryan Manning's corner, with Armstrong confidently finding the top corner.
The Saints almost doubled their lead nine minutes later when a terrific bending effort from Azaz was kept out by another good save by Crocombe.
Millwall were still threatening, however, and had a great chance to equalise when Ivanovic put a header from Femi Azeez's superb cross wide.
The Lions were level in the 72nd minute when Azeez exchanged passes with Crama before crashing in a low shot from just outside the area for his sixth goal in seven games.
The turnaround was completed in sensational fashion in the 81st minute when Taylor received the ball from Alfie Doughty before smashing a stunning strike into the top corner from almost 30 yards.
Southampton then thought they had snatched a point with three minutes left as Azaz squeezed in the rebound after Crocombe had saved Harwood-Bellis' header from Manning's free-kick.
But with almost the last kick, Crama was able to force Thierno Ballo's hanging cross over the line at the back post to spark rapturous celebrations among the home fans.
Millwall's Alex Neil:
"I thought the game itself was a brilliant game. It deserved an ending like it got and I feel we merited that across the 90 minutes.
"I thought we were good, we scored two brilliant goals - Tristan's being the most important, but the other two I thought were excellent in terms of the manner of them.
"I'm so proud of the players, so delighted for everybody connected with the club, because you look at the line-up and we've got a lot of younger lads in there that haven't had a lot of football.
"I thought they did brilliantly today as a group.
"I don't think I've ever run down a touchline as a coach, but I thought it was certainly one of those moments that merited it; a young centre-back pings one in on his fourth start, putting the ball into the top corner from 30 yards, so unfortunately I got a bit carried away."
Southampton interim boss Tonda Eckert:
"Every game, at the end, consists of small moments.
"I think we had the chance to go up in the first half, didn't manage to, then we did go up in the second half.
"I actually felt that, even though we defended a little bit deeper, we controlled them quite well.
"Then you know if you come here and concede a goal, the stadium comes on top and it's a lot of energy you have to put on the pitch to go against that.
"We conceded the second one and then compliments to the boys to get back in the game because I think that is very difficult, and then you just need to bring a game over the line.
"Millwall are a very good side and I think they had some moments when they came out on top.
"They just started to build momentum, but congratulations to them for (winning) the game."
