Not sure how helpful this will be to anyone who is having this problem on the 'End User Side', but for server admins and developers this may help narrow down the issue:
We were having this bug intermittently with our rss feeds on gluenow.com. An example of a feed is here:
http://sqd.gluenow.com/news/rss. I didn't find much online about this bug, aside from here... so I ended up just systematically removing different headers from our application and tweaking apache settings. Through this I did manage to fix this bug, but I am not sure why it fixes it.
Some background, Glue uses an apache front-end proxy with multiple mongrel clusters running the back-end Ruby on Rails app. Before, when the error would occur, our server was emmiting a Via: header, indicating that the content was proxied via gluenow.com. Here is what the headers looked like (with irrelevant headers removed):
rails-1 vhosts.d # curl -I
http://sqd.gluenow.com/news/rss
...
Via: 1.1 www.gluenow.com
Vary: Accept-Encoding
It seems that the "Via: 1.1 www.gluenow.com" header was causing the problem. I disabled the Via header in apache ( ProxyVia Off ), and now it works 100% of the time with no error.
So, at any rate, there are some examples in this thread that have the same issue, I would be interested if this workaround would fix them as well, or if they are even using the Via header right now (have not checked yet).
Hope this helps narrow down the cause of this issue.