Code in a renderer should never refer to global $PAGE. The page this renderer is rendering should be accessed as $this->page.

Similarly, never refer to global $OUTPUT. If you need to access the core renderer from a plugin renderer, use $this->output.

99% of the time this does not matter, because each script is only rendering its own output. However, in theory it should be possible to write a script like to archive all quiz results:

foreach ($quizattempts as $attempt) {

    $reviewpage = new moodle_page();

    // Setup $reviewpage;

    $output = $reviewpage->get_renderer('mod_quiz');

    $html = $output->render_review_page($attempt);

    // Store $html in a zip file.

}

If renderers refer to the global variables, this breaks .

We really ought to have a CodeChecker rule to catch people making this mistake, since it is clearly very easy to do.