Quote Originally Posted by bobwysiwyg View Post
It almost looks as though the sheet is riding up in the holder towards the open end. Could pulling the slide when shooting allow the sheet to ride up and thus a small strip never gets exposed? It seems unlikely. I've just tried a couple of the holders, one of which I'm sure was an offender, and there does not seem to be any way a sheet could ride up over the stops at the slide opening end. Any thoughts, suggestions would be appreciated. Thanks.
The film "Riding up" would have been my first guess. But if you are satisfied that you can rule out that possibility, perhaps you are not fully removing the slide when shooting.

I have a few sheets of 4x5 film from my beginner days when I did just that. For some reason, that was completely logical at the time, I thought it clever to leave the dark slide just barely in the film holder. I was bracketing and not concerned with composition or focusing at that point; just the time needed to wrap up the shot.

Needless to say, I never repeated that mistake.