Put Server->debugging->Debug messages to ALL: Show all reasonable PHP debug messages
and Server->debugging->Display debug messages to yes.
Define a tag in your profile. Save your profile and go to view it.
Click on your just added tag link.
By adding the youtube block to the page:
you get the message:
Notice: Only variable references should be returned by reference in moodle19/lib/phpxml/xml.php on line 73