[From "Homemade Rootbeer, Soda, and Pop" by Stephen Cresswell (ISBN: 1580170528) page 97]:
“Those who are experienced in making homebrewed beers know that one telltale sign of an infected batch is a ring around the neck of the bottle. While such a ring may also mean your soft drink batch has been spoiled by bacteria or other unwanted nasties, you should recognize that with soft drinks such rings can also be caused by bits of root floating to the top of the bottle, or by floating bits of citrus pulp. Thus, taste will be a better indicator of a bad batch than any ring round the neck of the bottle. Whatever caused the ring, though, be sure to clean the bottle with a bottle brush and then soak in a solution of water and chlorine bleach before brewing again.”
So, those rings could very well be sassafras, especially given how fine some of the bark that I added was. We’ll just have to wait until to tonight to see when I open the first bottle to check carbonation. I’ll keep my fingers crossed just in case though.