Just like the most of pseudo-XML tools, many RSS generators are buggy. For example, many just put the text into a CDATA section, without checking if it contains the sequence "]]>". As result, the feed might become invalid.
Recently I got a nice compaint from a tohtml.com user. He spent several hours to find why his feed was invalid. Finally he found that he highlighted XML with a CDATA section and got "]]>", which broke the RSS.
I've followed the suggestion and introduced a workaround: my syntax highlighter now returns "]]>" instead of "]]>".