Update: See comments below, and completely ignore this post. Nothing to see here...
module_load_include() is a great way to add code from other module's include files, but it doesn't always work as you'd expect. Recently, I was building a form in one module that pulled up a validation function from another module when a particular submit button was pressed:
$form['actions']['submit'] = array(
'#type' => 'submit',
'#value' => t('Awesome Submit Button'),
'#validate' => array('another_module_form_validate_function'),