Petr Cech endured a miserable Premier League debut for Arsenal as West Ham ended an eight-year wait for a win over their London rivals with a surprise 2-0 victory at the Emirates Stadium.
Cech was billed as the player who could make the difference for Arsene Wenger's side as they attempt to be crowned champions for the first time in 12 years this season, but the former Chelsea goalkeeper was culpable for both West Ham goals on Sunday.
The Czech Republic international flapped at a Dimitri Payet free-kick to allow Cheikhou Kouyate to head home just before half-time and reacted slowly to a long-range Mauro Zarate strike after the break that doubled West Ham's lead.
There was no way back for the home side in their first top-flight game of the season, as West Ham claimed their first victory over Arsenal in 16 attempts stretching back to 2007.
While Cech was left red-faced, Slaven Bilic celebrated an unexpected three points in his first Premier League game in charge, just three days after he was criticised for fielding a weakened side as West Ham crashed out of the Europa League with a defeat against Astra.
Mathieu Debuchy made his first Premier League start since January in the absence of Hector Bellerin (thigh), while Alexis Sanchez was named among the substitutes as he builds up his fitness after inspiring Chile to Copa America glory.
West Ham midfielder Reece Oxford became the second-youngster player after Jose Baxter to start a Premier League game at the age of 16, with new signings Payet and Angelo Ogbonna also making their top-flight debuts.
Alex Oxlade-Chamberlain, Arsenal's match-winner against Chelsea in the Community Shield last weekend, let fly with a rasping volley that flashed over the crossbar 10 minutes in.
Oxlade-Chamberlain then surged down the right at full tilt and delivered a cross beyond the far post into the path of Santi Cazorla, but his cushioned volley landed on the roof of the net.
Arsenal were in the ascendancy as Aaron Ramsey's strike from just outside the penalty area looped up and struck the crossbar after deflecting off Aaron Cresswell just after the half-hour mark.
It was West Ham who were in front three minutes before half-time, though, as the usually commanding Cech had a moment to forget.
The former Chelsea goalkeeper came out to try and punch clear a Payet free-kick from the right, but Kouyate beat him to it and hit the back of an empty Arsenal net with a towering header.
Olivier Giroud spun and fired into the side-netting and then forced an impressive stop from Adrian as Arsenal strived for an equaliser at the start of the second half.
But the home side's task became even greater 12 minutes after the break, when Zarate turned sharply and unleashed a long-range strike that beat Cech at his near post.
Wenger sent on Sanchez after 67 minutes and teenager Oxford was given a great ovation by the travelling fans when he was replaced by Kevin Nolan 12 minutes from time.
Cazorla's long-range drive was kept out by Adrian and Giroud shot high and wide as Arsenal looked short of ideas and in need of additional firepower, with the Hammers holding on to claim all three points.