If you paste
XF:Option's "Format parameters" for a 'php callback' option, instead of a nice error it instead gives;
Fatal error: Cannot declare class XF\Option\UserGroup, because the name is already in use in /var/www/html/src/XF/Option/UserGroup.php on line 0
\in the classname path should probably be treated as an error so the class loader system doesn't render random parts of the forum inaccessible. I assume this affects things like class extensions, code event listeners, etc.