When commenting out some block that has injections they are still processed and injected leading to invalid XML. Example: <title>Automatic Indexing</title><xref linkend="sect-Rebuilding_the_Index"/> <para> ... </para>
Fixed in 0.33.7 Injections will now only be processed if they are standalone (the only content in the XML content is the injection details).
This was released as part of 1.0.0