Troubles with un-subscribing for some users

@otto42 has managed to track down why we’ve seen topics about users unsubscribing from plugin forums but still receiving emails.

We’ve been seeing a lot of these: https://wordpress.org/support/topic/cannot-unsubscribe-from-plugin-support-page/

Short version: They unsubbed from a plugin forum, but still get emails, and the forum also does not show up in their subscriptions list.

Finally, I was able to get it to happen where I could see it happening. Seems that there is a desync between the database and the memcache for subscriptions, sometimes. Dunno what causes it as of yet.

Quick fix for anybody reporting this problem: You, the moderator, or anybody else really, can go and subscribe to that plugin forum that they are having trouble with, and then go ahead and unsubscribe from it. This invalidates the memcache entry and will cause it to rebuild from the database. All subs will be essentially “reset” to what they should be for that forum entry.

If I can sort out the cause, I’ll fix it. Until then, this workaround is easy to do.