Recurring error message when editing a post in WP 2.7 CR1

I love the new dashboard! Unfortunately, a recurring error comes up when I edited existing posts: it always returns a blank page with a single box with the message “You attempted to edit a post that doesn’t exist. Perhaps it was deleted?”

Can anyone explain this strange error, which is always incorrect? The page will pop up with the error message, but the post will have been properly edited.