How to change error-message displayed @ failed form-element validation to be different than the element's '#title" text?
Using a basic form in a custom module:

    function customform_demo($form, $form_state) {
    $form['test'] = array(
        '#type' =&gt; 'checkbox',
        '#title'    =&gt; t('This is the text that gets displayed next to the checkbox'),
        '#required' =&gt; TRUE,
    );

    $form['test_submit'] = array(
        '#type' =&gt; 'submit',
        '#value' =&gt; t('Submit'),
    );

    return $form;

}


You can set a custom error message in the form validation function. Here is a basic example with some comments:

    function customform_demo_validate($form, &amp;$form_state) {

    // Test for existing form errors
    if (form_get_errors()) {

        // Save errors
        $form_errors = form_get_errors();
        $drupal_errors = drupal_get_messages('error');

        // Clear form errors
        form_clear_error();

        foreach($drupal_errors['error'] as $key =&gt; $error) {
            if (in_array($error, $form_errors)) {
                // Unset form errors
                unset($drupal_errors['error'][$key]);
            }
        }

        // Rebuild drupal errors
        foreach($drupal_errors['error'] as $key =&gt; $message) {
            drupal_set_message($message, 'error');
        }

        // Validate form field and set error message    
        if ( $form_state['values']['test'] === 0 ) {
            form_set_error('test', t('You forgot to check the required checkbox. Please try again.'));
        }

                // Add more validation tests and error messages

    }
}