There was loads of bother when we were in Frankfurt for the Dortmund game a couple of seasons ago. Thought we were goin to get a kicking as soon as we left our hotel. Groups of Frankfurt fans dressed all in black(and all oddly wearing Sketchers trainers) attacking Dortmund fans in the streets around the station.
During the train and walk to the stadium it felt like it could kick off at any time even with a big police presence.