It’s “Content-Type” not “Content-type”
I accidentally discovered some of my feeds were not validating, not because of content, but because of the headers!
Upon inspection, I noticed some of them were being formatted nicely as RSS feeds and others looked like a dump of text or html.
It was driving me crazy, started blaming all sorts of things like eaccelerator and/or wp-cache.
Guess what, it comes down to a single uppercase/lowercase typo that a search found in a few plugins and some older RSS templates I was using from pre WP 2.1
If you find anything sending “Content-type” (note “type” vs “Type”) then some browsers, readers, feeders will break and not know what that means. Why? Because the RFC2616 specification says the case matters and some programmers follow the rules without allowing room for others that (accidentally) don’t.
You’ll need a good search/replace tool that’s case-sensitive. I use this old program and have been too lazy to see if there are any replacements around. It works fairly well so there’s never been a need.
One latent bug fixed, 1000 more to go… sigh…