Sorry I guess I should have been more clear - I thought the problem would be readily apparent (I was hoping it was something relatively common). It is an RB67 and I have taken hundreds of pictures with it and this is the first frame it has happened on. It is happening on only one frame of this roll. It is the frame with the trees (on the left in both pictures). You can see there is some kind of dark region on the emulsion that is roughly the shape of the frame, but not quite lined up with it. In the first picture you can see the dark region extends down to the edge of the film in the corner. In the second picture you can see the dark region does not extend up quite as far as the top of the image, and is clearly changing the density of the image when compared to the small strip at the top without the dark region behind it.