Between Sitecore 6.5 and Sitecore 6.6 Sitecore changed the way that Text Fields are rendered.They changed it so that text fields output was HTML Encoded. This is technically correct, and it’d be fair to call this a bug fix. It’s much safer to prevent users injecting HTML.
However, this is a problem for systems where users have put HTML content into those text fields, especially where you have thousands and thousands of pages.
Identifying where those fields contain such values could be … challenging. Instead, Martin English has written a nice processor for the RenderFields pipeline that should restore the old unencoded HTML output.