Sometimes when I'm browsing the All Items section, clicking on an item's title won't show its content. It happens often when I'm browsing using the J and K keys. I'm able to go through a number of items, but at some point I hit J and the next item doesn't display, and from that point on I can't expand any of the items in the list, including the ones I just read, and the J and K keys stop working.
Sometimes I'm able to manually browse to each of the feeds with unread items and read them, but not always. Reloading Reader on a new tab doesn't help. I think I tried clearing the cache and cookies when it first happened and it didn't work, but I'll try again after I post this. It has happened on Chrome (19.0.1084.56 m) and Firefox (8.0.1).