I did mention the film type- it's Fuji NPS 160. Rolleiflexes take 120. It's always possible it's a processing issue, but the lab that does my C41 for me runs dip-n-dunk, so it's unlikely to be a processing issue. I suppose it COULD be a problem with the film, as the film is about 11 years out of date. But that wouldn't explain why it happened to the middle of a single frame in literally the middle of the roll.