Answer to warning message, "First argument is expected to be a valid callback, '(some routine name)' was given in menu.inc"

If you are getting this message, or a similar message, on your site's content pages:

warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'views_arg_load' was given in /home1/sitebase/public_html/sitename/includes/menu.inc on line 407.

warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'views_access' was given in /home1/sitebase/public_html/sitename/includes/menu.inc on line 452.

warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'content_access_node_page_access' was given in /home1/sitebase/public_html/sitename/includes/menu.inc on line 452.

(and a Google query shows many people are seeing this kind of message)

...then here's help for fixing the problem. Look for a module that was active but is no longer active. Usually, the module name will appear as the first part of the named item that "was given". For example, if it says,

"valid callback, 'content_access_node_page_access' was given"

...then the module it was looking to call (and couldn't find) was the content_access module.

Check to see which modules you might have deactivated improperly (via a site move or database restore?), or that might have been deleted or moved from the modules directory. Reactivate or reinstall the missing modules to clear away the message.