St. Mary's was rocking as Jason Puncheon put the Saints ahead six minutes in, and it was two cataclysmic errors by Joe Hart and then Gareth Barry which gave Southampton a deserved win.
But what lessons did we learn from the game, for both sides?
A lot has been made of Southampton's inability to convert leads into wins in the past few weeks (via The Guardian).
When Manchester City pulled a goal back just before halftime, that sinking feeling began to reverberate around St. Mary's as fans worried that their team may surrender their two-goal advantage.
However, a combination of City's lack of cutting edge and Southampton's resilience saw the home team not just cling on to their lead, but add to it, with a Gareth Barry own goal securing an important three points for the Saints.
With Carlos Tevez not traveling down to the South Coast on Saturday reportedly due to family reasons (via The Guardian) and with Mancini's men light on attacking options thanks to the departure of Mario Balotelli, they went with Edin Dzeko and Sergio Aguero up front and no recognized striker on the bench.
Manchester City probably didn't expect to be 3-1 to Southampton after 47 minutes, but they did not have a forward to bring on and try to change the game once they were. Something which Carlos Tevez can do—and has done—on numerous occasions this season (via Soccerbase).
His work ethic and dogged determination is one which cannot be argued against, even by the biggest Carlos critics. And when your team needs two goals, he is a man who you would love to bring on from the bench, and he was missed on Saturday.
Mauricio Pochettino has brought some Argentine flair to Southampton since his arrival as manager.
He has started to influence the Saints' style of play in the same way that Roberto Martinez has done at Wigan. Their free-flowing football was a joy to watch today, and even Jason Puncheon showed some skill with some beautiful Cruyff turns and back heels.
Southampton are becoming irrepressible at home, collecting five points from their three Premier League home games since the turn of the year. And Pochettino has seen his new side draw with Everton, Arsenal and Wigan, beat Manchester City and only narrowly lose to Manchester United in the past month (via Soccerbase).
Despite the sacking of Nigel Adkins, raising more than just eyebrows (via Daily Mail), Pochettino has won over fans and neutrals, as Southampton are becoming a team which people now enjoy watching. A feat which Roberto Martinez has also achieved at Wigan.
Perhaps it is something to do with playing in Spain?
That epic double save against Brazil is but a distant memory for Joe Hart tonight, as the fate which seems to plague England 'keepers has finally caught up with the City man.
First, it was Calamity James, then it was "that" goal for the USA against Rob Green and now Joe Hart has had a nightmare himself, albeit on the club stage this time around.
After some delightful skill from Rickie Lambert, the Saints' frontman hit a low shot straight at Joe Hart, who should have been able to collect it reasonably easily for a goalkeeper of his talent. But he didn't, he fumbled the ball and let it spill straight into the path of Steven Davis, who couldn't miss.
You had to feel sorry for Joe Hart when you saw the look on his face as he scrambled back in attempt to deny the onrushing Davis.
With fans and critics looking to blame a person or persons after a poor display, there is one man who stands head and shoulders above the rest for the accolade of "scapegoat." No, not Joe Hart, but Gareth Barry.
Barry looked slow and clumsy for the majority of the game, and it was Jason Puncheon's easy dispossession of the midfielder which led to Southampton's first goal. However, his second error was even more damning as his attempt to steer the ball away from goal saw him, in fact, steer it into the back of the net.
His decision-making was confusing. There were no Saints players near him, there was no immediate danger and he had plenty of time to control the ball and clear it upfield. His teammates didn't even have a chance to tell him to take it easy before he slotted the ball coolly past his own goalkeeper.
The team as a whole were poor, but the leader of the pack was definitely Mr. Barry.
That clock will not be saying 00 years for much longer come May.
Manchester City are nine points behind their neighbors in red, and Manchester United have a game in hand—for the next 24 hours at least (via BBC).
Of course, United are not completely out of sight. Lest we forget last season, when City reduced an eight-point deficit with just six games remaining (via Soccer Association). But should United win on Sunday, that gap will grow to 12 points, and City's uphill climb to retain their Premier League crown could well be too steep.
With Sir Alex Ferguson's team still to face Arsenal, Chelsea and, of course, Manchester City, there is still a chance United could drop points on all of those games (via BBC). But if City play like they did against Southampton in their upcoming fixtures, it won't matter what results United get, as their title hopes will be dead and buried.