I thought that beans were strictly self-pollinating...until this year! While shelling my dry beans, I noticed some that looked like this:
I'm not really sure about the parents of the first one, but this is my guess:
Momma (Black Runner) - Baby - Daddy (Rattlesnake)
I remember picking the speckled "baby" beans from the Black Runner plant and the pods appeared to have faint stripes similar to the Rattlesnake beans.
The second "new" bean came from this pod on the Painted Lady Runner bean plant:
You can, again, see the purple stripes on the pod coming from Rattlesnake (aka "baby daddy"):
Painted Lady Runner beans
Offspring of Painted Lady and Rattlesnake.