"Viva Colonia!" on repeat. Ear-splitting Schunkel-Sound, a ska-schlager bastard of a genre that should have never seen the light of day. Fifty thousand people in a Kölsch-fuelled state of bliss. Lukas Podolski waving his hands at the crowd, egging them on. Tears of joy in the eyes of Dirk Lottner, the assistant coach and local legend. It was, in many ways, business as usual in RheinEnergieStadion, the cathedral of all-year-round carnival.

"Humour is laughing in spite of it" goes a German proverb. In Cologne, they've taken that [frightening] motto to its logical extreme. They'll dance where others would lie down and die, celebrate the uncelebratable, party at the drop of a hat, no excuse needed. But there was a marked difference on Saturday. For once, the crowd actually had a reason to go mental. Kölner Express called it the "Miracle of Müngersdorf" (after the ground's old name) and a "Lederhosen-Takeoff-Fairytale", Karl-Heinz Rummenigge called it "laughable". Christian Eichner had no idea what to call it. "I just don't get it," said the Köln defender, "it's not normal."

Louis van Gaal said it was "unbelievable" but he's been saying that all season. Actually, the most incomprehensible thing about Bayern Munich's second-half collapse after a 2-0 lead at the break was its eerie predictability. "It has happened before, in Gladbach, Leverkusen, Schalke, Wolfsburg and I don't know where else," said an angry Rummenigge. The champions, for whom a 1-0 lead used to be synonymous with a win, have become world-class wasters away from home: no one throws it away like them this season. In the Champions League game at Roma, the Bavarians managed to lose after scoring two goals in the first half. The last time they pulled that trick off in the league was in 1997, in Leverkusen.

And now against Köln, a team whose one outstanding feature is the sheer size of its limitations. Bayern, it is true, were fortunate to have Holger Badstuber still on the pitch early on, when the defender hacked down Milivoje Novakovic and prevented a clear goal-scoring opportunity. The rest, though, had been cake-walk. Bayern passed it around slowly, patiently. They didn't need more than a half a chance to score two goals. (Mario Gómez, Hamit Altintop). It could have been three – ref Felix Zwayer didn't spot Youssef Mohamad's handball in the box. It didn't seem to matter much. Gómez, confident of the win, sarcastically offered up his shirt to the Lebanese defender on the way into the tunnel.

In Köln's dressing room, their coach Frank Schaefer told his men they were at a crossroads. "You can let yourself go and lose 3-0, the way we did at St Pauli [last week]", said Eichner. "Or you can pay back the supporters for their fantastic support and go for it." Schaefer knew Bayern wouldn't be able to exert this much control over 90 minutes but even he couldn't have foreseen the visitor's total loss of concentration. An error-strewn 18-minute spell helped Köln turn it around in impressive fashion. Bayern only started playing football again once Novakovic's second strike had given the home side the lead (73) but by then, it was too late. "We came out like sleeping tablets [after the break]," said Thomas Müller, "we're destroying our whole season that way." Instead of moving up to second place, Bayern slumped to fifth.

In the stands, the official "Triumvirate" of Cologne's carnival season were having a field day. Prinz Frank I was there. So was Farmer Günter, an accountant in real life, and Virgin Reni, a real-life father of seven. Neutral observers didn't seem quite sure if the players in white might not be impostors, too. "Is that really Bayern Munich?", asked Frankfurter Allgemeine Zeitung.

Van Gaal, in uncharacteristic introspective mood, wondered whether his relaxed team-talk was to blame. "Maybe I should have taken Badstuber off", he added. The Germany international did have a nightmare but wasn't helped by the even more disastrous mistakes of Anatoly Tymoshchuk, who was doing his darn best to prove to Van Gaal that he's anything but a centre-back.

No one really knows how to explain Bayern's second-half flaccidness. Van Gaal's curious penchant for the kind of numerous, risky positional changes that you're more expected to find in a flock of desert bats during heat season might have something do with it though. Bastian Schweinsteiger (behind the strikers, not in defensive midfield), Danijel Pranjic (defensive midfield, not on the left or on the bench), Müller (right or left, not behind the strikers) and Tymoshchuk are all forced to moonlight in unfamiliar territory. There's still enough quality on the pitch to win most games despite this confusion but not enough stability to withstand the slightest bout of headwind.

"I'm shocked", said Van Gaal. Uli Hoeness, ominously, said nothing. The president had warned of consequences – for the manager – if the minimum target, qualification for the Champions League, was in danger. After the latest overdose of sedatives, that time has (almost) come.

Talking points

• Forty-odd years ago, Borussia Mönchengladbach were ruling the Bundesliga roost and frightening Europe alongside Bayern. The two team's fortunes have since varied wildly but on Saturday night, Michael Frontzeck's men showed that they could do exactly what Bayern did, only better: On top of throwing away a 2-0 half-time lead (against Stuttgart) and losing the match 3-2, they went the extra mile by doing so in front of their own shell-shocked supporters. Unlike Bayern, though, they could at least point to a Dowdyesque performance from the referee Thorsten Kinhöfer who wrongly ruled out a third goal for the home side and awarded the Swabians a late, contentious penalty for the win. "We got it right in the neck, it's a tough blow," said Frontzeck.

• "Manuel is the best goalkeeper in the world. That's good for Germany but bad for Dortmund." Little needs to be added to Mats Hummels's appraisal of Manuel Neuer after the goalless draw in the Ruhr derby on Friday night. Borussia were utterly dominant but frustrated by Schalke's hero between the sticks. Five times he pulled off terrific saves, twice the 2011 champions hit the post. "We didn't really deserve to get a point, that's for sure," Neuer said honestly. The result offered a tiny respite for S04 coach Felix Magath, who has enraged his own fans with a flurry of nebulous transfers. "At Schalke, it's important to get the supporters on your side," warned the club chief executive Clemens Toennies. "[Magath] didn't do that. It's his own fault he was criticised." Magath has admitted to his mistakes ("I should have talked more to the fans") but denies there is a problem. "Everything is going according to plan." He's right, of course – if the plan entailed slipping dangerously close to the relegation places.

• No goals either at the other big derby, Hamburg v St Pauli. This time, however, it wasn't goalkeeping heroics but gardening blunders that kept the sheets blank: the newly relaid pitch in the Volksparkstadion couldn't cope with the rain and the match was called off. In a bizarre bet, the HSV captain Heiko Westermann and the deputy St Pauli captain Gerald Asamoah had agreed that the loser would clean the winner's car. Topless.

• In the Volkswagen town of Wolfsburg, they might well have mooted a similar punishment for Diego but at the end of the day, it's cold, hard cash they prefer. The Brazilian midfielder will reportedly be fined €100,000 and possibly banned for taking (and missing) a penalty against the express wishes of Steve McClaren. The new striker Patrick Helmes was supposed to take it but was pushed aside by Diego, who himself had been muscled out by Edin Dzeko earlier in the season. "This is unacceptable," thundered McClaren. "I'm very angry. This matter had a direct influence on the outcome of the game." Wolfsburg lost 1-0 away to Hannover and are only one point clear of the play-off relegation place. "I'll have to think about things," the general manager Dieter Hoeness replied when asked about the Yorkshireman's job prospects. Translation: One more defeat and for you, ze game is up, Stefan.

Results: Borussia Dortmund 0-0 Schalke, Hoffenheim 3-2 Kaiserslautern, Nürnberg 1-0 Bayer Leverkusen, Mainz 1-1 Werder Bremen, Hannover 1-0 Wolfsburg, Köln 3-2 Bayern Munich, Gladbach 2-3 Stuttgart, Freiburg 0-0 Eintracht Frankfurt.

Latest Bundesliga table

• After this blog was published, Steve McClaren was sacked by Wolfsburg. You can read more about his dismissal here.