File for menu callback not always loaded if it is for a form

[mglaman]

  $items['examples/form_example/wizard'] = array(
    'title' => 'Extensible wizard example',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('form_example_wizard'),
    'access callback' => TRUE,
    'description' => 'A general approach to a wizard multistep form.',
    'file' => 'form_example_wizard.inc',
  );

When submitting the form, it fails that form_example_wizard cannot be found.