I'm inexperienced at this, and hoping the answer to this question is something obvious. I was doing a stand development protocol with Rodinal, 1:90. 30 inversions initially, then a 75 minutes stand. I've done this 5 or 10 times so far, with various films and pretty good results. This time I got a brown stain encroaching from the edge of the film roll. I was developing 2 rolls at once (I've done that before), and the stain was on the roll that was on top. I'm 99.9% sure this wasn't a volume issue, because the tank calls for 600 mL of fluid when doing two rolls, and I had 650 or a little more. So I think the film was adequately covered. I am positive this roll was the top one, and I assume (but can't say for sure) that the brown edge was the upper edge. Anyone have any ideas what might have caused this?