It was always 'stain. I remember from my childhood. And while it would be cool if the universe was somehow being rearranged, the reality of this situation is far more simple. Popele can raed jbumeld wrods if the fsrit and lsat few lteetrs are rhgit. It's a shortcut our brains use to make us read faster. Our parents read Berenstain, and the aforementioned mental shortcut caused them to process it as Berenstein, a much more familiar sounding name, and an easy one to assume. We, in turn, had the same oversight. I know this because when my mom read me the books, I always thought Berenstain was a funny sounding name, and it seems like Berenstein would make more sense.