I got a bunch of tiny black specks on a roll of 35mm Pan-F. I had mixed the developer with our well water. I read somewhere on another thread that a small amount of dissolved Iron salts in the water can cause this with certain developers and films.
I mixed another batch of developer with distilled water and there were no more black specks. The negs looked fine. Sometime later I tried filtering our well water with a water filtration pitcher, and that worked also.