I do not know if any one had this experience. I sowed plum tomato seeds from a store bought packet and one of the plants is producing round tomatoes.The fruits are still green but they look nice and firm. The other plants are producing plum tomatoes.
I think Ecrane has hit the nail on the head, just enjoy and IF you save seeds make sure you store them in different packets so you know the seeds from the plumb tomato's are just that.
Good luck. WeeNel.
The same thing happened to me last summer. Except I had a variety of beefsteak tomato and one of my plants grew plum tomatoes. It's better to have a variety of plants growing near each other to stave off disease anyway.