Petr Cech made a Premier League debut to forget for Arsenal as his errors helped West Ham United to record a 2-0 victory at the Emirates Stadium on Sunday.
Cech's arrival from Chelsea has been acclaimed as a signing that could turn Arsenal into title contenders, but he was found wanting as Arsene Wenger's side opened their Premier League campaign with a home defeat for the second time in three years.
Cech's rash charge allowed Cheikhou Kouyate to head West Ham in front from debutant Dimitri Payet's 43rd-minute free-kick and the Czech international's slowness to get down saw him beaten at his near post from 25 yards by Mauro Zarate shortly before the hour.
"We were not convincing offensively or defensively," said Wenger, whose side beat champions Chelsea 1-0 in the Community Shield last weekend.
"The players were maybe too nervous and put too much pressure on themselves. Today we have been hurt mentally and it is a good opportunity to respond."
West Ham's win echoed Aston Villa's 3-1 success at the Emirates on the first day of the 2013-14 campaign and enabled Slaven Bilic to crown his first league game as manager with a famous victory.
"We are all very pleased and it is a great start for us - not only for the three points, but points against a team that are title contenders," said the Croatian, who is a former West Ham defender.
"If you add to that a London derby, it makes it an even bigger win. This is very special. I am very proud of the second-half performance."
Wijnaldum scores on debut
Bilic named 16-year-old Reece Oxford in midfield and saw the teenager turn in a commanding display as West Ham bounced back from their Europa League elimination at the hands of Romanians Astra Giurgiu last Thursday.
Wenger sent on Theo Walcott and Alexis Sanchez as Arsenal sought to claw back their two-goal deficit, but the closest they came to scoring was a deflected first-half shot by Aaron Ramsey that hit the bar.
Meanwhile, Dutch midfielder Giorginio Wijnaldum scored on his competitive Newcastle United debut as former England head coach Steve McClaren began his St James' Park tenure with a 2-2 draw at home to Southampton.
After Graziano Pelle had put the visitors in front, goals from Papiss Cisse and 14.5 million-pound new recruit Wijnaldum put Newcastle in front, but Southampton substitute Shane Long headed in a 79th-minute equaliser.
Liverpool launch their season later on Sunday at Stoke City, scene of their chastening 6-1 defeat on the last day of last season, while Manchester City travel to West Bromwich Albion on Monday.
Chelsea opened their title defence with a 2-2 draw at home to Swansea City on Saturday, with Manchester United edging Tottenham Hotspur 1-0.