(CNN) -- Borussia Dortmund's German title defense suffered a big blow on Saturday with a controversial 3-2 home defeat by Wolfsburg.
It left Jurgen Klopp's third-placed team, seeking a third successive Bundesliga crown, 14 points behind runaway leaders Bayern Munich.
The match turned in the 35th minute when referee Wolfgang Stark ruled that Marcel Schmelzer handled the ball on the line, despite it appearing to hit him in the leg.
Stark sent off the Dortmund defender and awarded a penalty that Diego converted to level the score at 1-1.
"I've looked at it again later and unfortunately it was an error of perception on my part," Stark told reporters.
"I'm sorry, that should not happen. The penalty and the red card were a mistake on my part. That's annoying."
Klopp was left to rue the consequences, as struggling Wolfsburg went on to win with goals from Naldo and Bas Dost -- whose shot had earlier hit Schmelzer.
"We were 1-0 ahead and Wolfsburg had no chance and then comes the game-winning move. It was a brutal decision," Klopp told reporters.
Bayern won 2-0 at Augsburg to move 11 points clear of second-placed Bayer Leverkusen, who travel to Hanover on Sunday.
Thomas Muller opened the scoring six minutes before halftime with a penalty, his eighth goal in the league this season, after Gibril Sankoh handled Toni Kroos' pass.
Substitute Mario Gomez continued his comeback from longterm injury with the second soon after his introduction on the hour mark. The Germany striker followed up his midweek goal in the Champions League win over BATE Borisov for his third strike since starting his season late last month.
Eintracht Frankfurt joined Dortmund on 27 points from 16 matches with a 4-1 win over Werder Bremen, while Schalke dropped to fifth after losing 3-1 at Stuttgart.
Vedad Ibisevic scored a hat-trick as Stuttgart joined Schalke on 25 points, taking the Bosnia striker's tally to 13 goals in all competitions this season.
Freiburg moved up to eighth with a 1-0 win over bottom club Furth, while Nuremberg stayed above Wolfsburg on goal difference after beating fellow strugglers Fortuna Dusseldorf 2-0
Hamburg ended the day seventh, having won 2-0 at home to Hoffenheim on Friday.