Inexplicable mistakes by Joe Hart and Gareth Barry gave Manchester City's Premier League title hopes a potentially decisive blow as Southampton secured its first win under Mauricio Pochettino in style, 3-1 on Saturday.
City goalkeeper Hart admitted in the build-up to the game that anything but victory at St. Mary's would all but end City's hopes of retaining the title, with leaders Manchester United already boasting a nine-point cushion.
Roberto Mancini's side simply did not look like one in need of a victory Saturday, conceding after just seven minutes when Jason Puncheon slotted home.
Hart could have possibly done better with the shot that led to the opener, but there was no doubting his culpability for Southampton's second.
Edin Dzeko reduced the deficit as halftime approached, though a two-goal lead could have been restored on the stroke of halftime had referee Martin Atkinson not waved away Jay Rodriguez's penalty appeals.
That was soon forgotten, though, as Barry passed into his own goal unchallenged to put Saints on course for a first win since Pochettino replaced Nigel Adkins as manager.
On the opening day of the season, the newly promoted side was valiant in a 3-2 defeat at the Etihad Stadium.
It was a similarly end-to-end encounter on Saturday, with Saints dominating the proceedings early on and netting a seventh-minute opener.
After Barry was dispossessed, Puncheon burst down the right flank and played through the unmarked Rodriguez. Hart managed to deny the winger but the ball fell kindly for Puncheon, who slotted home calmly in front of the Northam End.
City was struggling to deal with Southampton's attacking intent in an opening 15 minutes in which a tame Sergio Aguero effort was all it had to show for its efforts.
City eventually began to settle but was soon rocked by a second Southampton goal in the 22nd minute.
Lambert collected the ball on the right and played a neat one-two with Puncheon, before cutting back and firing a shot that Hart let slip through from his midriff, allowing Davis to poke home.
Saints fans ironically bellowed "England No.1" after the gaffe, before directing their attention at referee Atkinson.
First they took umbrage with his decision to book Lambert for hauling back Joleon Lescott, before he then awarded a foul on the edge of the box after Dzeko took a tumble.
Rodriguez saw a right-foot effort go just inches wide of Hart's far post as the home side pressed for a third, with Nathaniel Clyne seeing a strike deflect behind.
Pablo Zabaleta's floated ball to the back post just evaded Aguero as City looked for a goal, which came six minutes before halftime.
The Bosnian striker got himself in the way of a terrible Saints corner before bursting up the field and tucking home a Zabaleta cross.
It was harsh on the hosts, who felt hard done by in stoppage-time when Rodriguez was brought to ground in the area by Yaya Toure, captaining the side on his return from the African Nations Cup.
Atkinson awarded a corner instead of a spot kick -- much to the home fans' frustrations -- but they were celebrating soon after the restart.
After Puncheon hammered over moments into the second half, the winger could then not quite latch onto a Lambert pullback, which Barry inexplicably passed into his own goal.
Gael Clichy was booked soon after as City's frustrations became evident, with Saints pushing for a fourth.
The visitors were having to defend with all they had and hope for a chance on the counter, with Artur Boruc reacting well to palm away an Aguero strike wide when they did voyage forward.
Aleksandar Kolarov and Maicon were City's final introductions, with the latter soon denied from close range by Boruc.
James Ward-Prowse replaced Davis for a nervy final 10 minutes in which City was kept at bay, meaning United could go 12 points clear at the top with a win against Everton on Sunday.