Arsenal came up short in their first really serious test of the season on Tuesday as Borussia Dortmund brought the flying Gunners down to earth with a jolt.
Arsene Wenger's men have been playing fantasy football in recent weeks as an unbeaten 12-match run took them to the summit of the Premier League and top of their Champions League group.
Against Norwich City at the weekend fans were left drooling at the quality of Arsenal's midfield of Mesut Ozil, Jack Wilshere and Santi Cazorla with in-form Aaron Ramsey coming off the bench later.
The first goal against Norwich would have graced Barcelona at their peak, however, Dortmund on Tuesday were a completely different proposition and were in no mood to stand and watch Wenger's men go through their repertoire.
The Bundesliga side who reached the Champions League final last season were powerful, organised, quick on the break and ruthless when opportunities arrived.
Unlike most sides they have faced so far in a relatively kind fixture list in the Premier League, Dortmund stifled Arsenal for long periods but still managed to play high up the pitch, forcing Arsenal's creators such as Ozil into ever tighter spaces. Arsenal's title rivals would have been taking notes.
It was only when Cazorla came on for Wilshere after an hour that Arsenal began to find some spaces but Robert Lewandowski's 82nd minute volley ensured that Dortmund returned home with all the points to put them back in the frame in Group F.
"Technically we have met some good teams this season but physically they were stronger than many other teams that we have met," Wenger said of Dortmund on Tuesday.
Dortmund were well organised to stop us from playing and we put ourselves on the back foot with the first goal and were naive for the second goal."
Olivier Giroud's equaliser just before halftime after Henrikh Mkhitaryan's opener looked to have given Arsenal a deserved point from an intense game but they now find themselves in a real dogfight to qualify for the last 16.
Some may suggest that Arsenal have been found out, that they lack the mettle for a real title challenge in the Premier League. That would be harsh, although what Tuesday did show was the importance of squad depth.
Mathieu Flamini was unavailable with concussion against Dortmund and there was no obvious replacement for the no-nonsense Frenchman who has done such a great job in a deep-sitting role in Arsenal's midfield.
Without him Arsenal never really got a grip and their playmakers were starved of the ball.
Setbacks can make teams stronger, however, and all eyes now will be on how Arsenal recover from Tuesday's kick in the teeth.
Playing at struggling Crystal Palace this weekend should allow Arsenal to shake-off any lingering disappointment but tougher tests are looming with Liverpool and Manchester United in the league either side of the return in Dortmund.
That sequence will offer a more realistic clue to whether Arsenal really are on the verge of something special.
"It's going to be tough," said midfielder Mikel Arteta. "The toughest group in the Champions League and we're still capable of going through but we didn't deserve to lose the game."