Randomly, I get a subscribed thread marked as having an unread post; incorrectly.
I double check, but sure enough there really is no new posting. This weirdness persists for a while sometimes and then goes away mysteriously.
A ha!
I get that occasionally too. I choose to ignore it because it sounds like something that once I start investigating I will regret. You can ignore it too. 
For certain. I was just adding it to the queue if there is one. But I wholeheartedly agree, it screams “regrettability high, importance extremely low”.
Maybe I should have put a “reply unnecessary” tag!
I think it has something to do with SPAM posts being deleted. If a new post is made in a thread, it puts the thread into “new unread post”. Then if that post is deleted, when you go to the thread it looks like there is no new post - which indeed there no longer is. Yet for some reason the deletion doesn’t seem to clear the “new unread post” status of the thread - at least not right away.