Peaches and Nectarines cross-pollinating?
 

I've never had this happen, but this year, two of our peach trees have produced several - very big and beautiful - nectarines along with the peach crops. With one, I just happened to notice two nectarines on the ground by my peach tree and wondered where they came from. This tree has small, just developing peaches on it. The nectarines are large and ripe. Then I saw several more on the peach tree.

Today, I was mowing, and noticed several nectarines on a peach tree that has already finished producing. (At a record rate for this tree)
We only have one true nectarine tree. And it only had a couple of blossoms this year. And no fruit.

I'm just curious. How often does this happen? I've had fruit trees for years and have never seen this on my trees.