The whole post hinges on the statement:

"When MongoDB is all you have, it’s a cache with no backing store behind it. It will become inconsistent. Not eventually consistent — just plain, flat-out inconsistent, for all time."

OK, here is a chance to share a little insight. At what point did Mongo become hopelessly inconsistent? Were you ever able to determine why? Why bother with cute pictures and verbose explanations of simple schemas when the conclusion is just that Mongo breaks no matter what without further explanation?