It's easy to forget and make silly mistakes.....I recently set out to reload a camera which I hadn't used for a few months, opened the back and found a part-exposed film in it (as it happens, HP5+). Totally forgot ! I remembered that the shots used were of no importance, so took the film out, wound it back to the start noted the number of exposures made, and kept it until I needed a short length of B&W to shoot. Obviously, when I later shot the remainder of film and processed it, the fogged part was exactly as the OP's example. Mistakes happen to us all ?