This section implies that by using the !$may_cache case in hook_menu() in a custom module it is possible to arbitrarily overwrite existing menu item definitions.

However, while it is possible to do this using the $may_cache case and by increasing the custom module's weight in the system table so that it is called after the module originally providing the menu path, only certain elements of the menu item can be overwritten by using the !$may_cache trick.

For example, while the callback can be overwritten, the 'access' element cannot, hence the line:
'access' => user_access('access devel information')
is actually redundant.

See http://drupal.org/node/189384#comment-625981 et seq. for more info; this also references the relevant code in menu.inc.