The following 3-1 win confirmed their place several steps higher than the rest of Europe that year. Their 4-0 defeat to Bayern Munich on Tuesday night showed that they no longer stand alone.
You have to delve back nearly a full six years into the archives for the last time Barcelona lost by four or more goals.
Leading Getafe 5-2 in the Copa del Rey, they were hammered 4-0 and made a shock exit from the competition. You can't help but feel that defeat in the Allianz Arena was more poignant.
With statistics from Who Scored and Squawka allowing us to instantly make our own analysis, matches streamed online worldwide and open forums like Twitter to express opinions, there is no hiding when things go wrong anymore.
Barcelona have enjoyed the highs—they are possibly the first team to truly blossom in the mass Internet era—and now they have to deal with the lows.
The defeat against Bayern Munich doesn't start the debate, though. It merely strengthens it.
Already Real Madrid have suggested that they have the beating of Barca each time they meet, AC Milan surprised them at the San Siro and PSG remained unbeaten against them over two matches.
While they opted to sign Alex Song last summer and continue to be linked with Neymar, they defend as though the concept is foreign to them—possibly because they've had to do so little over the last five years.
Fingers will point to the absence of Carles Puyol and Javier Mascherano, but the defensive frailties have long been in the making—namely since their attacking has begun to pack less of a punch.
Lionel Messi (was he fit?) wasn't allowed a sniff. UEFA's postmatch stats revealed Andres Iniesta managed to pass directly to him just three times. Pedro Rodriguez and Alexis Sanchez didn't offer a goal threat, unlike how Thomas Muller so effectively does for the Germans, and Barca's two best chances fell to rookie defender Marc Bartra.
Possession remains their name—they had 66 percent in Munich (via whoscored.com)—but the transitional change of pace in the final third is no longer their game:
GRAPHIC: Barca heat map Vs Bayern. Didn't get close to the goal...: twitter.com/Squawka/status…— Squawka Football(@Squawka) April 23, 2013
They are still set to clinch La Liga in the coming weeks, although it will feel like the most diluted title they have won in recent years.
In this Internet era, the Champions League has a tendency to trump everything else.
Barcelona's defeats to Celtic, AC Milan and now Bayern Munich, as well as the draws with PSG, prove the complete dominance they have enjoyed in recent years is over.
They haven't moved forward. They're standing still and other clubs have caught up with them. It happens.
Among Europe's elite, La Blaugrana remain one of the best—six consecutive European Cup semifinals is no fluke—but they no longer carry the weight that propelled them to top billing on every tactic's website.
If Barcelona want to dismiss talk of the end of their dynasty as premature, they'll have to do what no team has done before.
Real Madrid came from 5-1 down to beat Borussia Monchengladbach in 1985, and the previous year Partizan Belgrade overturned a 6-2 defeat to QPR to progress. But no team has ever rallied from 4-0 down in the competition (via @2010MisterChip).