If your negatives have crescent-shaped dense marks (they appear lighter in the positive/printed image), the cause is generally mis-handling of the film while loading the spiral. If you mechanically stress the film, the halides become activated and will therefore develop as if they've been exposed.
This is most common with 120 film due to its width; if one side of the film sticks in the spiral while you're winding it on and it is forced, there is a significant shear force across the film and it will crinkle.
Once this has happened to a film, there is no cure for it. The most-reliable way to prevent handling damage like this is to make sure that spirals are absolutely bone-dry by drying them with a hot hairdryer immediately before use, even if the spiral was already dry.
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