You sometimes get that with roll film, too. It happens when the film presses hard up against the guides in the tank, and the processing chemicals can't get to it. Sheet film is thicker, so it may be more likely with the 2509 tank, although I haven't seen it so far. It shouldn't extend into the image area. (If it does, it's some other problem.) I should be harmless, but you can refix in a tray, as recommended above, if it bothers you.