options_field_settings_form() includes :
$form['allowed_values_function_display'] = array(
'#type' => 'item',
'#title' => t('Allowed values list'),
'#markup' => t('The value of this field is being determined by the %function function and may not be changed.', array('%function' => $settings['allowed_values_function'])),
'#access' => !empty($settings['allowed_values_function']),
);This only intends to output some informational markup in the form, but ends up putting an empty string for 'allowed_values_function_display' in $form_state['values']. This then gets saved back to the CMI file for the field, and dummy configuration schema snippets have been added for this.