Okay, this may sound strange but one of my russet potatoes has produced an acorn shaped fruit from its branches. I touched it and it fell off so I'm sitting here looking at it. It is green and hard and about the size of a small tomatillo or large cherry tomato. The plants are at the flowering stage, what the heck is this thing?

Those things contain the actual seed. Here's a pretty good site to explain it:
http://www.growseed.org/potato-breeding.html

I've had these from time to time.

Wow, I had no idea! And to think I used to live in Florida's Potato Capital--Hastings. Thanks alot hon!

Do not eat this fruit!