I recently started using my friend's mom's old Canon 35mm film camera. I was a bit annoyed when having the film developed by a lab, and copied to standard photo paper - there is a single horizontal discoloring on a lot of the photos, consistent with what i see when people have dust on their scanner head.
However, the line is present not only on my scans, but the physical copy as well. I then thought it might be down to the film stock, or something in the development process that's out of my hands. Imagine my surprise when i find the same (or an eerily similar line) on a completely different roll of film, developed by another shop!
The only common denominator at this point is the camera. Could it be causing this somehow? I recall the polaroid mantra "clean your rollers", and there is indeed a few wheels guiding the film.
As per comments, i went and had a look at the film itself. The dia slides look physically damaged (the line is visible when viewed at an angle, reflecting light in a different way). This is not the case for the negative film, where i can't see the line at all (could still be there but couldn't make it out through a loupe). Thus the two lines may be unrelated.
Follows two scanned examples:
And a phone photo of a slide from the other roll (dia film), projected on a wall: