User1 has selected to receive notifications by email. When User2 tries to reply to topic User1 is subscribed to, he gets error:
Fatal error: Call to undefined method phpbb_notification_method_email::notify_using_messenger() in [snip]/includes/notification/method/email.php on line 50
To reproduce it install phpBB, subscribe to topic phpBB has created by default, check option to receive notifications by email. Create new account and try to post reply to that topic.