Looks like a light leak to me from the top of the camera or back somewhere. The most likely place is usually the back, so open it (or flip that folding part out on an M camera) and see if there's something wrong the w/ the seal or the metal on that door or the camera's body. Light leaks are not always consistent. Depends on where the sun is at, where you have your hands at on the camera, which way it's pointing when you took the shot, etc.