Sam Allardyce had a birthday to forget as his West Ham side put on a defensive horror show to gift Manchester City their first away win of the season.
A complete breakdown in communication between James Tomkins and Winston Reid allowed Sergio Aguero to put City ahead, and slack marking from Ricardo Vaz Te later allowed the Argentinian to head home his seventh of the season.
Vaz Te made amends slightly by scoring with an acrobatic overhead kick, but David Silva scored 10 minutes from time to give Manuel Pellegrini's men a 3-1 win.
The Chilean manager could be happy with his City team's performance. Silva, Aguero and Alvaro Negredo were all outstanding, but their task was made much easier thanks to the Hammers' poor defensive showing.
Allardyce, who turned 59 on Saturday, had prided himself on the fact that West Ham had only conceded five goals in their first seven games, but on this occasion they were poor at the back.
Silva was afforded way too much space every time he collected the ball while Reid and Tomkins were easily outdone for pace by the excellent Aguero.
Pellegrini admitted City's away form was a concern before kick-off. Their performance in east London was not on a par with their rip-roaring display against Newcastle, but it would be foolish to write off their title hopes, especially when their neighbours, Manchester United, have endured such a difficult start to the season.
Hopes were high among the home fans at kick-off. With the 3-0 win at Tottenham still fresh in their minds, rarely-heard chants of "Sam Allardyce's barmy army" rang out around Upton Park.
West Ham started with the same purpose and drive they displayed at White Hart Lane two weeks previously.
Mohamed Diame picked the ball up from Razvan Rat before cutting inside and hitting a swirling shot that flew inches wide of Joe Hart's goal.
Hart gave the Hammers fans further reason for optimism when he mis-hit a clearance, but the home side failed to take advantage.
Then City started to take a grip of the game.
Reid put in a crucial tackle to deny Aguero but the New Zealand defender switched off moments later and City's Argentina striker took full advantage.
Fernandinho played a long ball along the floor from the halfway line towards Negredo. Reid and Tomkins expected the Spaniard to collect the ball, but he sold them a dummy, leaving the ball completely. Aguero was in behind the two defenders in a flash and he slotted the ball calmly past Jussi Jaaskelainen.
Aguero could have made it 2-0 two minutes later, but he volleyed straight at the West Ham goalkeeper while free inside the box.
City were playing some delightful football. Silva was given way too much time on the ball and he kept looking to unlock the home defence with a series of superb incisive passes.
Aguero was looking unstoppable. The striker slalomed his way through the West Ham back line and drew another save from Jaaskelainen.
The home crowd groaned as Ravel Morrison attempted a spectacular turn which backfired.
Silva then entered the book with a dangerous late tackle on Rat. Michael Oliver cautioned Kevin Nolan for a much less harmful challenge.
West Ham were barely creating anything going forward. Vaz Te headed weakly into Hart's arms just before half-time.
City flew up the other end and would have doubled their lead had Jaaskelainen not pulled off a top save to deny Yaya Toure.
What Allardyce said to his team at half-time had little effect as the Hammers fell 2-0 down after another piece of shambolic defending.
This time Vaz Te was the offender. The Portuguese player gave Aguero two yards of free space in the box and the striker capitalised by nodding Silva's cross past Jaaskelainen.
Allardyce shook his head and buried it in his hands, but Vaz Te put himself back in favour by putting the Hammers back into the game.
Rat's cross made its way to the back post where Vaz Te, with his back to goal, leapt up and acrobatically beat Hart from eight yards.
The goal lifted the home side, but City went back on the attack with Negredo, who struck the bar on a breakneck counter.
Allardyce made two changes, with Matt Jarvis and Joey O' Brien coming on for Guy Demel, who had picked up an injury, and Vaz Te.
West Ham were vulnerable at the back as they pushed for an equaliser and City capitalised.
With 10 minutes left, the visitors went on a lightning counter-attack. Toure played in Aguero, who backheeled to Silva and he curled the ball home after giving Reid the slip.
The fans who hailed Allardyce at kick-off flooded out of Upton Park, and City relaxed, knowing full well the three points were in the bag.