The Gunners stumble as teams around them make up ground
It was an intriguing Sunday in the world of sport, although an early red card at The Emirates Stadium spoiled what could have been an even more interesting game.
From red cards and getting on the right side of tight results (well, apart from the 6-0 a couple of years ago), Chelsea know what it takes to get past Arsenal.
And this evening, with both sides having different ambitions for the season, the same result transpired.
Arsenal's vulnerability when it comes to balls over the top of the defence had the end result of seeing Mertesacker sent off as the slow-paced defender's challenge saw Diego Costa go to ground with acres of space between the Gunners back-line and goalkeeper Petr Cech.
A couple of old failings showed up for Arsenal when opportunities did arise at the other end with a snatching of chances and occasionally too many intricate passes being played before getting a shot away.
However, a positive for Arsene Wenger is the fact they did not cave in at 1-0 and one man down, although that might have to do with Chelsea not going for the kill.
More worryingly, a potent looking Tottenham are just two points behind them in a four-horse race for the Premier League title, and Wenger will rue the late concession of Joe Allen's late goal for Liverpool in the 3-3 draw from earlier this month which could have given Arsenal a greater cushion in the table.
Ulster were always going to be reliant on a couple of results going their way today in their hopes of a Champions Cup quarter-final place.
But as it happened, Stade Francais got the bonus point win they needed to get into the Top 8 sides for the pool stages.
All of which means the Irish provinces collectively miss out on the European knockout stages for the first time since the 1997-98 season in the old Heineken Cup.
Leinster and Munster in particular suffered major setbacks, with Leo Cullen's side bottom of their tough pool after a single win in six games.
Stung by Wasps home and away with a total points difference of -68 from those two games, it was a chastening experience for the three-time champions.
Ulster got closest to the knockout stages as they were left ruing home and away losses to Saracens.
There had been worries that the French and English sides would start asseting even more of a stranglehold on the Champions Cup due to the huge wages they can pay and five Premiership teams and three Top 14 sides in the last-8 tells its own story.