Well, it's probably just because of some processing snafu of mine back in the mid-1990's, but I have a paper negative (one of my nicer pinhole images, BTW), shot in 8"x8" format onto Ilford MG-RC-III, that now has a kind of 'grunge' happening inside the paper (or perhaps between the emulsion and the paper?) A surface scan looks normal; however, when trying to contact print this negative the outer portion of the printed image prints lighter, due to the increased density internal to the negative. The 'grunge' is most evident near the edge, and fades in toward the middle of the negative. You can easily see the problem when viewing the image back-lit.
I can't easily blame anything or anyone but myself; perhaps I didn't rinse adequately - or rinsed too long, which is also a problem with RC, as after leaching of liquid into the paper it's harder to remove than with fiber based. But after seeing this happening to some of my favorite images, it really has me concerned that perhaps my paper negative work should be transfered over to fiber-based negatives.