Quoted from ItsOnlyPinball:Oddly enough, I saw the black streaks on the balls from the spinning disc on the Kiss I played in Minneapolis, but not yet on the one in Madison. Possibly because Hilton and the crew changed out the balls to a higher quality? I would suspect the one in Minnie has the original balls and not sure if this has made a difference or not.
The one at Mortimer's and has a ton of plays already. It's the same issue as Tron when it first came out with the black rubber marks/pieces everywhere. Stern said to use rubbing alcohol on the disc and that they were aware of the issue, it doesn't have to do with the balls. D&G removed the rubber disc completely in the meantime as it was really gunking up the game.