Southampton - Arsenal's Premier League title ambitions were dealt a painful blow as they were not so much beaten as humiliated 4-0 by a Shane Long-inspired Southampton on Saturday.
Favourites to win the title after their 2-1 victory over Manchester City, they knew that they could go to the top of the table by beating an out-of-form Southampton after Leicester City's defeat at Liverpool earlier in the day.
But somehow they made Southampton - short of confidence and goals and reeling from a 2-0 home defeat by Tottenham Hotspur a week ago - look like the team aiming for the top.
Dutch-born Curacao defender Cuco Martina, only in the team thanks to an injury to first-choice right-back Cedric Soares, put Southampton ahead with a sensational long-range strike in the 19th minute.
Long, himself only playing because of injury to top scorer Graziano Pelle, added a second, Jose Fonte headed in the third, and Long made it 4-0 in injury time.
St Mary's is not a lucky ground for Arsenal, where they lost on New Year's Day last season and have not won since December 2003.
But this was a potentially damaging result and display and Arsene Wenger's men must now aim for six points and nothing less from home matches against Bournemouth and Newcastle United.
Yet Arsenal started well. Unchanged from the starting XI that had beaten City last Monday, the visitors could have gone ahead in the second minute, but Olivier Giroud headed the ball well over from Mesut Ozil's corner.
Southampton looked energetic and in the mood to put recent disappointments behind them, but their early attacks foundered on the Arsenal defence.
When the visitors broke away, Aaron Ramsey's clever disguised pass reached Nacho Monreal, whose deflected cross almost drifted over the head of Maarten Stekelenburg.
Monreal had another chance when Martina headed unconvincingly out of defence and the left-back under-hit his shot straight at Stekelenburg.
But that error by the man from Rotterdam was soon forgotten.
When Per Mertesacker headed away a routine cross from the left, any danger to the Arsenal goal seemed to have been averted until Martina hit the ball first time with the outside of his right foot, sending it curling past Petr Cech and into the bottom-left corner from fully 30 yards.
No-one looked more surprised than Martina, who was making his first league start after being restricted to a couple of Europa League appearances since his £1 million close-season move from FC Twente.
Southampton could have added a second three minutes before half-time when Long outpaced Mertesacker, but with only Cech to beat, lifted his shot high over the bar.
Seconds earlier Theo Walcott had nodded wide at the near post from another Ozil corner, but Arsenal's previous dominance was a thing of the past, and things got even worse in the opening period of the second half.
When James Ward-Prowse sent a free-kick onto the head of Virgil Van Dijk, only the flag of the assistant referee ruled out a goal. It was a close decision.
But Southampton were not to be denied and in the 55th minute they deservedly doubled their lead.
Long began the move, passing to Mane on the right and running into the box for the return pass. Laurent Koscielny, his marker, tripped after fleeting contact with Long, who was left unmarked to shoot past Cech.
It was 3-0 in the 69th minute as Southampton scored a goal that summed up everything wrong with Arsenal on the night.
As Ryan Bertrand swung in a corner from the right, no Arsenal defender attacked the ball convincingly, in contrast to Saints captain Fonte, who threw himself between Koscielny and Mathieu Flamini and headed past Cech.
Now it was just a question of how many Southampton would score and having gone close when he hit the post, Long added a fourth when he drilled home a deserved second.