Manchester City's title challenge suffered near-fatal damage in a calamitous 3-1 defeat to Southampton that witnessed a hilarious own goal from Gareth Barry and a terrible error from Joe Hart.
City went behind after only seven minutes when Jason Puncheon profited from a Barry mistake but it was the nature of the following two goals that ensured the champions' trip to St Mary's would be remembered as a comedy of errors.
Hart embarrassingly let a shot slip out of his hands after 22 minutes, allowing Steven Davis to tap home from close range, before Barry then met a Rickie Lambert cross in the second half with a perfect, measured side-footed finish into the corner of his own goal.
City had already pulled one back when Edin Dzeko started and finished a fine breakaway goal, but an insipid performance meant they could not prevent a first Premier League victory for Mauricio Pochettino which takes Southampton 15th, six points clear of relegation.
Meanwhile, an awful evening for City leaves them nine points behind leaders and rivals Manchester United, who can extend that lead to 12 and effectively end the title race with a victory against Everton on Sunday.
City, who pushed Javi Garcia back into defence as Yaya Toure made his return from the African Cup of Nations, had to do without Carlos Tevez as he withdrew close to kick-off due to family reasons, so Roberto Mancini was without a striker on the bench.
On a cold and wet evening in Southampton, the reigning champions were desperately disappointing from kick-off and found themselves a goal down inside seven minutes. Puncheon robbed a culpable Barry of possession and fed Jay Rodriguez. He was denied by Hart but the ball popped out to Puncheon, who had continued his run into the box, and the winger tucked the finish away.
His celebration - which involved squatting and a wiping motion - referenced Puncheon's infamous need to answer a call of nature during a draw with Everton in January. It could just as easily have been a prophetic judgement on Hart's goalkeeping for the second Southampton goal though.
Rickie Lambert cut inside Gael Clichy after 22 minutes and though his shot was firm, it was hardly dangerous. Hart, though, somehow contrived to drop the ball and Davis followed up to tap the ball home and punish England's number one.
Southampton's high-tempo approach left City looking distinctly subdued and Rodriguez had a fine chance to make it 3-0 when the visiting defence failed to deal with his initial incursion into the box and after initially losing possession he then curled an effort just wide of the far post.
Saints were bristling with confidence, but that spilled over into cockiness on 39 minutes and they paid for it. Attempting to emulate the Beckham-Scholes corner routine, Danny Fox was shaping to execute a volley from the edge of the box when the ball was nicked away from him by the head of Dzeko. After David Silva then slipped in Pablo Zabaleta on the break it was the striker who met his cross with a stabbed finish that accidentally lifted the ball past Artur Boruc.
City, though, failed to be lifted by the goal and indeed should have been punished in first-half injury time when Toure clearly clattered into Rodriguez in the box and brought him down. Bizarrely, referee Martin Atkinson chose not to point to the spot.
An improved second-half performance was expected from City but their evening quickly took a turn for the worse. Minutes after Puncheon had slammed a shot over the bar following a neat piece of hold-up play from Lambert, the latter delivered a cross from the left and, under little pressure, Barry bizarrely side-footed the ball into the corner of his own net. It was utterly perplexing.
City tried to respond when Aguero latched onto a ball over the top from Barry - this time attacking the correct end - but when he did get a shot away on the spin, Boruc clawed it out with a quite brilliant save to his left.
Mancini sought to change things up and removed Joleon Lescott for Aleksandar Kolarov. The substitution meant the Serb joined Clichy and Zabaleta in a back three with Garcia pushing on in midfielder and City were left without a single centre-back on the pitch. The Italian then took off Silva in order to introduce Maicon.
Neither worked. A City team who were possibly surrendering their title continued to look leggy and uninterested, while Southampton's zesty energy carried them to the most unexpected of victories.
MAN OF THE MATCH
Rickie Lambert (Southampton) - Led the line manfully and didn't give the City defence a moment's rest. It was his shot that Hart fumbled for the second goal and his 'assist' for Barry too.
SOUTHAMPTON: Boruc 8, Clyne 7, Hooiveld 7, Yoshida 8, Fox 7, Cork 7, Schneiderlin 8, Puncheon 8, Davis 7, Rodriguez 7, Lambert 8. Subs: Lallana 7, Richardson 7, Ward-Prowse 7.
MANCHESTER CITY: Hart 5, Zabaleta 5, Garcia 4, Lescott 5, Clichy 5, Toure 5, Barry 4, Nasri 4, Silva 6, Aguero 5, Dzeko 6. Subs: Milner 6, Kolarov 5, Maicon 5.