I'm having issues with fetch_rss not properly unserializing a cached feed (both feeds from flickr and delicious). I noticed the issue when I attempted to use the rss-stream plugin.
http://api.flickr.com/services/feeds/photos_public.gne?id=21209393@N04&lang=en-us&format=rss_200 -- this (I believe) is the url of the flickr stream.
It is in an issue in both 2.5 and 2.6.
