After performing a company search in /companies/some.php, if the user clicks 'New Company', the following warning appears at the bottom of /companies/new.php:
Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively in Unknown on line 0
The problem was actually caused by the arr_vars_session_set() function in utils-misc.php. Wholesale assignment of values led to some unset values being assigned to the $_SESSION extension which triggered the warning. All good now :)