This is what happened when I had a loose roll in my pinhole camera: apart from the expected fogging at the edges, there's also an image of the numbers on the backing paper, although I can't explain how. (Ilford FP4+, so near-black on off-white printing on the paper, and decent quality normally light-tight paper).

http://farm4.static.flickr.com/3239/...047_z.jpg?zz=1