One of my users reported the following error message after creating a new post on notifications enabled content:
Undefined property: Notifications_Node_Post_Event::$notif_error in Notifications_Event->send_list() (line 555 of /sites/all/modules/notifications/notifications.event.inc).
The class declares $notif_errors, so the problem here is simply a typo. Patch will follow.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.