My XML feed for Trulia found here doesn't seem to render correctly. But upon inspection all tags appear to be correctly nested and I don't see any errors with syntax.
W3C's feed validator outputs this report, saying that the feed isn't well-formed, but it seems correctly formed to me?
column 39: Undefined root element: properties
column 35473: XML parsing error: <unknown>:1:35497: not well-formed (invalid token)
Escaping illegal characters by wrapping them with CDDATA in the format <![CDATA[foo]]> cleared one of the issues, however this wasn't the only reason my feed wasn't being read.
The problem was that the MIME type for the feed was text/html instead of xml. I checked the MIME types configured on my server and saw no issues however.
I later discovered that it was a caching issue caused by a Wordpress plugin that operates in the same directory. By excluding that file in W3 Total Cache's whitelist the feed rendered correctly.