When, for some reason, sending an email to a user for a forum post notification (within the send_user_notification adhoc task) fails the whole adhoc task fails and gets retried after its fail delay. But since the system/web notification is sent before trying sending the email, the user gets the web notification over and over again.

While searching the tracker for an existing entry for this issue I just found this comment from simonyoung. But I think the issue deserves its own tracker entry.