Yes, the stain is on the non emulsion side and I now don't think it's a hardwater stain because the drip's widest point, when hung, is on the top so that the drip looks like it's pointing downward.
Lisa - I thing your conclusion is correct - the stain is not related to anything you are doing in your darkroom.
Years ago, I experienced a similar problem on bulk-loaded TMY - a stain on the non-emulsion side and appeared to be oily. I never found the cause.
In another instance, I had some strange black thingies that stuck to the emulsion side of the film. Clearly not dust or lint, and looked a bit like it could have been microscopic threads caused by a dull tool when the film was slit to width.
I also had a couple of bulk rolls that had a wierd blue cast (totally different from the magenta cast caused by incomplete fixing). I processed the film in two different darkrooms, and with totally different chemistries, and got the same result. Again, I never found a cause.
A conclusion is that the old adage "make one to print and one to scratch" really makes sense - always make at least two negatives of each scene as insurance against some kind of unanticipated problem.
If life didn't occasionally have a few mysteries, it would be awfully boring.
