Five things we learned about the Bundesliga

Here are five things we learned from the 21st round of matches in the Bundesliga:

Ancelotti's middle finger

Bayern Munich's mild-mannered coach Carlo Ancelotti let his calm demeanour slip by showing Hertha Berlin fans the middle finger, later claiming home supporters spat at him, following Saturday's 1-1 draw.

The German FA are set to open an investigation on Monday after the Italian admitted to the vulgar gesture as tempers frayed at Berlin's Olympic Stadium following Robert Lewandowski's 96th-minute winner.

Ancelotti could face a fine of around 3,500 euros ($3,715), based on the punishment handed out when FC Union's then-coach Norbert Duewel made a similar gesture in December 2014.

Misfiring Mueller

Germany forward Thomas Mueller's miserable run of form in front of goal continues for Bayern.

Having been left out of the starting line-up for Wednesday's spectacular 5-1 thrashing of Arsenal in the Champions League, Mueller started up front with Robert Lewandowski rested for Saturday's 1-1 draw at Hertha.

Mueller has scored just one league goal this season and did not managed a single shot with his 41 touches in Berlin.

"I don't know if it's good to keep talking about this," he tensely said when a Sky reporter asked him about his lack of form. "I give my all, try to score and to put in passes."

But the 27-year-old World Cup winner has no place in Ancelotti's first-choice line-up.

To rub salt in Mueller's wounded pride, Lewandowski came off the bench to hit Bayern's winner.

50,000th for Bellarabi

Germany winger Karem Bellarabi made Bundesliga history when he scored the league's 50,000th goal and his shirt is to become a museum piece.

His opening goal in Bayer Leverkusen's 3-1 win at Augsburg reached the milestone tally in the 54 years since the Bundesliga first started.

Bellarabi's shirt will become part of a permanent exhibition at the German Football Museum in Dortmund.

"It's a great thing and I'm proud to have achieved it," said Bellarabi, who also holds the record for the Bundesliga's fastest goal when he scored after just nine seconds against Borussia Dortmund in 2014.

Dortmund's terrace ban

Dortmund's iconic south stand, nicknamed the 'Gelbe Wand' (Yellow Wall), was an eerie and empty sight for Saturday's 3-0 home win over Wolfsburg.

The club locked out the 25,000 fans who normally pack the terrace for one game as part of a German FA punishment after hooligans attacked visiting RB Leipzig fans at the start of the month.

"Dortmund without the south stand is like football without a ball," said CEO's Hans-Joachim Watzke.

But the Dortmund boss is determined to ban those responsible for attacking RB fans, including families, and says the numbers of those set to be excluded for home games next season as a result will reach 'three figures'.

Krkic flops in Mainz

Wandering ex-Barcelona midfielder Bojan Krkic arrived with a long list of titles to his name when Mainz signed him on loan from Stoke City in January.

He won the 2011 and 2009 Champions League titles with Barca, plus three La Liga titles and the 2010 Club World Cup, and also lifted the Dutch league title in 2013/14 with Ajax.

But the 26-year-old, who has played one international for Spain, has been a flop in his three Mainz games so far in the attacking midfield role.

He drifted out of Saturday's 2-0 home defeat to Werder Bremen and was taken off with half an hour to play

He touched the ball just 29 times against Bremen without putting in a single dribbling run or dangerous pass.