Probably a small light leak or an internal reflection at the time of exposer. Many times light leaks aren't noticed on all frames is because your shooting sequence between frames is fairly short and the time the film actually spends for a particular frame in the gate area is short and the small leak just doesn't have enough time to work on the films emulsion to expose it. Other times you'll take a few shots and then the camera may set idle for a day or two or maybe much longer. thus, much more time for that tiny light leak to have a lasting impression on the films emulsion. You can do a quick test to see if you have a light leak by snapping a frame and then taking one of those nice super bright LED flash lights (the brighter....the better) and shinning it all over the camera from head to toe for several minutes. You can do this on several frames, but remember the frames so you can look at the negative/prints after processing. Also, remember that if it is a light leak that the camera exposes everything upside-down and backwards in the cameras film gate. So, as of your example shows a light leak in the lower right corner? That indicates a light leak in the upper left region of the camera. They (the leaks) on these cameras are almost always in the rear door area and might be a sign of a bad seal or crack in the plastic.
Now, for the other possibility? This is an older camera so I don't know if this was an option or not, but some cameras had a date/data printer built into the back that allowed one to imprint date and time on the negative. If this camera has that option it could also cause you a problem if it is not working correctly. You can solve this problem with a small piece of electrical tape to cover the printing slit under the pressure plate on the back.
Hope this helps since you have a very good camera with a very fine Tessar lens.........................................................John