On a day Arsenal boss Arsene Wenger was celebrating his 64th birthday, the Gunners were unable to deliver a beautifully wrapped birthday present to their manager.
Rather the north-London side who currently sit atop the Premier League, fell at the Emirates to last season's champions League runner-up.
Already, pessimists have started saying Arsenal had failed their first major test of the season when in reality Wenger's team were unfortunate to lose to one of Europe's best teams thanks to a goal by Robert Lewandowski, who by the way could have been shown the red for an elbow on Laurent Koscielny.
Arsenal need not be hard on themselves. This was the performance of a developing team, one that will suffer setbacks along the way. It must be pointed out though, that the developmental project is nearing completion before I get some backlash from pundits.
There are lessons that could be learnt from the clash and two of the most sobering lessons involved Wenger's most influential summer signings.
Matthieu Flamini was missing from Arsenal's line-up, having suffered a concussion against Norwich on Saturday. Arsenal missed his ability to shield the back four, take up crucial positions and the touch of ruthlessnes that he could have brought to the game which might have prevented two rather easy goals by Dortmund.
Arsenal when guilty of cheap concession of possession and slack marking in the build up to both goals.
The second lesson Arsenal can take from the clash has to do with their £42.4m siogning from real Madrid - Mesut Ozil.
On the night, Ozil was thwarted by a Bundesliga team and a German coach in Klopp who know him very well. The sleek midfielder was denied the much need space that could have allowed him foment trouble. Rather he was confined to small spaces and his influence was limited.
However, a conclusion cannot be drawn that if Ozil flounders Arsenal will fall. It is not a case of "stop Ozil and you stop Arsenal" as they have too many creators for that. It is a reality, though, that if you can subdue this graceful operator then you remove one of Wenger's most potent weapons.
On a positive note, Ozil was seen in flashes but one sublime pass he played through set up substitute Santi Cazorla for a shot that glanced off the woodwork at 1-1.
Some satisfaction should be drawn from the game by the Gunners fans as it wasn't altogether a bad display and as a matter of fact, Arsenal still possess the ability to bounce back stronger.