It might be caused by the overexposed part of the upper frame. The light that came almost directly in to the frame caused that kind of in-camera reflection, one also can see it on the left side, over the text. The sunlight cane in by the 3/4 right side (see the shadows) and was reflected by the subject, perhaps the window, directly into the lens. In the frame it self, the scattering of the light can be seen as a triangular shaped white spot in the fence and over the shadow-mark of the tree.
And there is always a little reflection inside the film-holder, just enough to cause a problem on an other frame.
This is what I suppose might be the problem, I hope it helps.
Good luck in 2009,
Philippe