I did that exact bug once. But I didn't just get one email for each subscriber, since the index didn't increase the loop never terminated. Took me some time to actually kill the process (PHP script running at some provider back in the day). Got over a 100k emails to my Gmail, so much that my account crashed and it took a few days before I managed to log in again (got an error saying something went wrong when opening gmail in the browser). So at least then it was very possible to ddos someone's mailbox.