Solution to nodereference_autocomplete_access error

We recently rolled out a site to testing and got a mysterious error:

warning: call_user_func_array() []: First argument is expected to be a valid callback, 'nodereference_autocomplete_access' was given in /var/www/html/drupal/includes/ on line 453.

We were using Acquia Drupal, so the nodereference module was right where one would expect. What was Drupal's problem?

Turned out to be totally unrelated. The developer had missed checking Drupal's .htaccess file into the repository, so this copy of Drupal was missing its .htaccess file, and all sorts of havoc (including the message above) resulted. The tip was when clicking on the login form resulted in a 404 Page Not Found, which led to the realization that clean URLs weren't working, which pointed the big fat Drupal finger directly at .htaccess problems.