While investigating a (probably) unrelated problem I noticed that the slack plugin does not post any kind of message to the error logs (https://site/logs) if the slack notification fails.
Tested by setting an invalid slack outbound webhook url on a site and posting a new topic as well as pressing ‘Test Notifications’.
This is one of the things I fixed with discourse-chat-integration, which hopefully will be replacing the slack plugin very soon - it writes errors to the site logs, and shows them in the admin interface next to the associated “channel” as well.
For example:
Nice, can’t wait for this!
FWIW, I also discovered that the existing ‘Test Notification’ button is a gamble - it isn’t guaranteed to send out a notification. It only does if the post it randomly selects matches the filters.
Yep, that’s another thing that’s fixed. discourse-chat-integration allows the user to select a specific topic to send, so there’s no chance of it trying to send a PM, and then failing 
