What is an example you have? It seems that it has historic posts as long as anyone else every has followed the feed via Reader previously. Likewise, it won't display old posts in the list, but will dynamically load them as you "try" to scroll down. Perhaps one of these is what would happen for you?
There isn't a specific example feed. It used to happen for all unread posts. I don't know if it still happens or not as I don't leave enough posts unread anymore.
The exact behavior was with several hundred unread posts in Reader with the oldest unread being 2-3 months old the oldest posts would slowly disappear. I know it was happening because they would be posts I wanted to read but didn't have time to so I would notice when they disappeared. It is possible they were still in Reader in a read state but I never bothered to check.
this is actually (apparently) a intentional behavior, old posts that you never personally marked unread will after a month or several be marked read. I only recently discovered this myself, and yes it is an intentional behavior. (how else would you be able to handle reading all the newer articles? :p)
No. It was not a specific blog. I explained the issue in more detail in child comment of your comments sibling. It has been months if not years since I last noticed it so Google may have fixed it.
