Smoke bombs were let off by supporters in the 46th minute of the clash at Signal Iduna Park, blocking the view for some of the travelling fans.
The DFB has taken a dim view of the incident and subsequently sanctioned Mainz for the disturbance.
Mainz, who were leading 1-0 at the time, went on to lose the match 4-2 - with Marco Reus playing a pivotal role with a goal and an assist for Jurgen Klopp's men.
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