This problem happens often but I haven’t found how to make it reproducible.
On the latest beta of the iOS 7 app, sometimes when reading several unread stories in ‘All stories’, if having the device in portrait I turn it to landscape it will occasionally switch to a different story I read previously. When it happens if I go back to the story I’m trying to read in portrait mode and turn the device again to landscape it will switch again to the other story. As far as I have noticed this seems to happen when a previously read story had a graphic in it that had been viewed in landscape mode.