Yesterday I noticed a change of the behavior of Google Reader in the 'Sort by Oldest' mode. I traditionally read from oldest to newest, since newer items typically refer to older items, I get the whole story. When I clicked on a feed, I immediately went to the oldest post that wasn't marked Read. Starting yesterday, when I click on a feed, I go to the oldest story cached in Google Reader, which are typically ones I've marked as Read already. This is really strange, and has completely screwed up my feed reading workflow. Did something change recently? I didn't see any additional options to fix this behavior.

You can switch between showing all items and showing new items. The option is on the bar right above news feed items, near the "mark all as read" button.