Today, my notification menu showed an unread reply on the icon on the right, but there is no unread reply notification in the menu.
At first, I thought maybe this happened when a user replied to me, and the user (including their post) was deleted before I read it. But when I reproduce that scenario, the notification disappears on reload. The notification here on Meta is sticky. It doesn’t disappear on reload and is still there after logging out and back in.
This feels similar to Can't clear unread notification after being removed from a PM, but so far I have no idea what caused this, and it’s pure guesswork because I have no clue what happened last night.