Well, I tested my holders this weekend, and I only have a couple with light leaks that would affect the photo. They seem to all have a tiny little slit leak right in the bottom corner where the flap meets the body of the holder though. Perhap the darkslides were not pushed all the way in? (I'd expect the whole bottom of the film to be affected though, and this is not the case)
The "bad" leaks are all along the bottom of the holder, either in the corners or along the bottom itself. I can see how some hockey tape would fix the leak along the bottom, but what do I do about the corners? I can't see where the light would be coming from. How are these usually fixed, while leaving the flap mobile and not adding any material to the face of the holder (which would thereby cause an uneven interface with the camera back)?