Leverkusen enjoyed a strong season last year and even bagged Champions League qualification after finishing third in the Bundesliga.
This year, their campaign has got off to a sour note after they were stunned in the opening round of the DFB Pokal by third tier SV Elversberg.
The 3.Liga outfit scored three first-half goals as they powered to a famous 4-3 victory over their top tier opponents.
Elversberg, who have never been higher in the German football pyramid than the third tier, rocked Leverkusen after just two minutes as Jannik Rochelt got on the scoresheet.
Bayer hit back through Adam Hlozek three minutes later, but they were soon behind once more, as Jean Koffi converted a first-half penalty.
Charles Aranguiz levelled the game in the 30th minute, but that didn't seem to discourage the hosts, who took the lead for the third time through Luca Schnellbacher.
Bayer struggled to find a way through Elversberg's steely defence, and the Bundesliga outfit soon paid the price as Kevin Conrad made it 4-2 with quarter-of-an-hour remaining.
Patrik Schick eventually found a breakthrough in the 89th minute, but his effort was nothing more than a conciliation goal as the hosts secured a famous win.