Southampton hammered Arsenal 4-0 with a remarkable performance at St Mary's.
Ronald Koeman's side had been badly out of form and few would have predicted such an emphatic win over the in-form Londoners, who had seen off title rivals Manchester City on Monday.
Here, we round up the talking points from the game...
Arsenal blow chance to go top
Throw out the form book. It doesn't seem to matter in the Premier League this season.
Going into the Boxing Day fixtures, Arsenal had won their last three in the Premier League, including Monday night's seemingly landmark victory over title rivals Manchester City. Southampton had slumped to five without a win, managing just one point - against bottom of the pile Aston Villa - in that streak.
Yet, despite scoring just three times in their last six in all competitions, Southampton fired four past the hapless Gunners. Saints had conceded 14 in their last half dozen games but barely gave Arsenal a sniff at goal.
While the most unpredictable Premier League season continues to throw up shocks and surprises, Arsenal will rue the fact they missed a golden opportunity to capitalise on Leicester's defeat at Liverpool and go top of the Premier League.
Manchester City's win over Sunderland pulls them back to within one-point of second-placed Arsenal, and Tottenham's triumph at Norwich means they are only four adrift of their north London rivals.
This was Arsenal's fourth defeat of the season. With trips to Liverpool, Manchester United, Tottenham, Everton and Manchester City to come in 2016, they can hardly afford any further slip-ups.
Arsenal's defence crumbles
This was Arsenal's heaviest defeat since their 6-0 thrashing at Chelsea in March 2014 - but it could have been a lot worse. Just days after showing heart and organisation as they held out in the final stages against Manchester City, the Gunners' backline crumbled at St Mary's.
Cuco Martina, Shane Long (two) and Jose Fonte got their names on the scoresheet but Long could easily have had a hat-trick had he controlled his first-half chip or got away with a physical challenge on Per Mertesacker in the second period, while Virgil van Dijk saw a header narrowly ruled out for offside.
Arsene Wenger later criticised referee calls on three of the goals - but conceded his team, which had previously had the joint-best defence in the division, had lost the physical battle to Southampton. "We lost too many challenges to win the game," he said.
Martina scores a wonder-strike
There can't be many better maiden Premier League goals than Cuco Martina's wonder strike to open the scoring at St Mary's.
The right-back - signed from FC Twente in the summer - was only making his first Premier League start but had the confidence to strike a sensational shot with the outside of his right-foot, on the half-volley from all of 30 yards. He gave Petr Cech no chance.
In doing so, Martina became the first player from Curaçao to score a Premier League goal - and surely put himself in the mix for the goal of the season prize.
Wanyama's midfield masterclass
Victor Wanyama produced a stellar defensive midfield display against Arsenal. The Kenyan made more tackles (six) and regained possession (10) more times than any other player on the pitch. According to Opta's tracking data, only team-mate Steven Davis covered more ground.
As January approaches, Wanyama is a reminder to Premier League clubs the best work in transfer windows can often be keeping hold of your current stars. Wanyama seemed set for the Southampton exit door when he handed in a transfer request last summer, with a reunion with Tottenham boss Mauricio Pochettino seemingly on the cards.
Saints boss Ronald Koeman was adamant Wanyama was staying put and, on Boxing Day, the player produced a performance which must have left Wenger - who has seen so many of his central midfielders hit by injury - envious.