The theme function looks a bit like this:
// Generate the output using either a function or a template.
if (isset($info['function'])) {
if (function_exists($info['function'])) {
$output = $info['function']($variables);
}
}
else {
..
// Render the output using the found template file.
$output = $render_function($template_file, $variables);
}
return $output;if $info['function'] doesn't exist, a notice will be thrown:
Notice: Undefined variable: output in theme() (line 932 of /home/nsh/drupal7/drupal/includes/theme.inc).