When adding the HTML_Block to a Database activity the page refused to load and following error was thrown up in debug:
Call to undefined function file_rewrite_pluginfile_urls() in /home/imoot/public_html/2011/blocks/html/block_html.php on line 60

This has been determined to be because lib/filelib.php hasn't been included in the block

I have listed this as critical as it impacts a core block that is rather heavily used