What happens if a dentist fills a cavity with some decay left on the tooth?

Needs to be redone: If there is decay present then the filling needs to be redone. The decay shoul be removed.

Some Decay: Ideally all clinical "decay" should be removed. There are isolated instances when if the decay is very close to the nerve, your dentist may leave a small amount, treating the area with a medicament to help the tooth to heal itself. If your tooth responds as desired, you will not need a root canal. If your tooth does not respond as desired then, you may require a root canal procedure.