Derek -
If you are using a semicircular pinhole camera (round tin, oatmeal box, etc.) that curves the film, and if you are using a film that has a shiny surface (no retouching texture on the emulsion side), the film may be reflecting light onto itself. I had this happen when using color transparency film in a large can-type camera. Tri-x has a somewhat matte surface on the emulsion side that allows for mechanical retouching, and has not caused this problem for me when used in a can-camera.

Peter Gomena