I had something very similar happen to me this year, too. In my row of Blooming Prairie, which is a low bushy pole plant with small lavender seeds, one plant grew enormously tall and made leaves as big as my face and big seeds like this:

Anyway, I think these new beans are outcrosses, and wonder if the pattern isn't the default/"wild-type" pattern for beans. I can't for the life of me find it right now but somewhere in the middle of this enormous thread everybody starts posting photos of their surprise beans, and a lot of them have the same streaky pattern as ours do.

Back in 2009 I planted a block of Fowler Bush, which is a normal sized bush bean with dark coffee colored seed. One plant in the bunch grew into a totally rampant pole bean. I provided a pole, and then a second pole and finally, yet another pole, to support this plant. It produced pods which looked close to Fowler, and the seed was also dark coffee colored. But the pods were not as tender.

I sent some seed to a fellow seed saver, who grew them out. The resulting plants were all pole beans. There were maroon colored seeds, Genuine Cornfield (tan with black swirl) colored seeds, dark coffee colored seeds and one other variation, which, at the moment, I can't remember. He sent me samples of all four variations. Through this I was able to determine that Fowler had crossed with Cherokee Striped Cornhill. I wanted to try growing at least one of these F2 selections. But, alas, I just never managed to make the time and space.

Crosses are not uncommon. I later learned that hot, dry weather, increases crossing between beans. When the plants are stressed their flowers open up more, allowing pollinating insects to spread the "love."

George
Tahlequah, OK

" hot, dry weather, increases crossing between beans"
We had a very dry summer this year, not the hot that you guys get but hot for here. I did bag a few flowers on most of the beans I grew this year and I have kept this seed separate but I won't be too surprised if a few crosses turn up the next time I grow these varieties.

I thought my 'Mr. Tung's' had crossed this year but I must have dropped a bean out of my pocket when weeding around this pole. It turned out to be a 'NCSLGCS'.

The only cross or ? I've ever had in all the years I've grown beans is from Mr. Tung's and it was only one bean in a pod of normal looking seed.
Top left, the bean in the middle.Top right, what this bean produced.
Bottom left, the shelly and dried seed the above bean produced. Bottom right, when I planted it another change. The pod the same size and shape but much more pink on it. I still have this seed but don't know if it's still viable.