The "check for updates" page within the ACP's System tab produces invalid XHTML when there is a new version available. This is caused by HTML entities in the URL linking to the announcement which are not encoded.

W3C validator:

Warning Line 266, Column 77: cannot generate system identifier for general entity "t"

…bb.com/community/viewtopic.php?f=95&t=1777775" title="http://www.phpbb.com/co