Bayern Munich slumped to a shock home defeat by Borussia Moenchengladbach in its opening Bundesliga match on Sunday following a blunder by its most expensive new signings.

Bayer Leverkusen, runner-up last year, also paid for lapses at the back as it lost 0-2 at Mainz.

Igor de Camargo scored the only goal in the 62nd minute for Moenchengladbach after Bayern goalkeeper Manuel Neuer and defender Jerome Boateng got into a terrible mix-up over a long ball forward by Roel Brouwers.

Neuer, signed from Schalke 04 amid protests from a group of Bayern fans angry at the purchase of a former rival, rushed out to try and clear the ball and former Manchester City defender Boateng left it for him.

But Belgian De Camargo nipped in to head the ball over the goalkeeper and give Moenchengladbach, which escaped relegation by the skin of its teeth last season, a shock win.

"Gladbach were very well-organized. They built a spider's web in which we always got caught," said Bayern coach Jupp Heynckes, starting his third stint in charge of the Bavarians. "The goal came from nowhere, it was a misunderstanding between Boateng and Neuer."

Bayern, which fired Louis van Gaal and replaced him with Heynckes in its bid to improve on last season's disappointing third place, dominated the first half without creating any real chances.