Fogging of some sport, by the initial look of it, or even maybe something wrong with the development of the film, though it look a bit regular for that. I only ever had the metal slide in place when I was going to change/remove the magazine; over many years of use neither of two bodies, 500C and 500CM, ever leaked, so no, it isn't a common fault. In fact, I doubt there were many common faults, in the sense of common meaning regular. Of course, as with everything, it depends on looking after your stuff over time, and since you were not the original owner, you won't be able to tell.
If the rest of the camera is in good condition, it's well worth fixing!
Rob C