I developed two rolls of c41 last night. One was an old roll of Kodak Pro Image 100 (obviously very expired) and the other was a new roll of Fuji Superia 800. Both were developed in the same tank at the same time. There is no visible staining or watermarks on the base or emulsion side of the negatives from either roll. I have developed a ton of color (including multiple rolls) and have never seen this before.
Here is an image from the Pro Image 100 roll:
And here is an image from the Fuji Superia roll:
All images from the Pro Image 100 roll came out fine. The images from the Superia, obviously, did not. (Although I think they look cool).