This is definitely a glitch that a lot of people have been having lately. I've had it happen to me a couple of times within the past month or so. What probably happened is that you got an alert, but you ended up checking it through other means (for example, if I got a blog comment and clicked on the blog to check it instead of clicking "view" in alerts), and the fact that you checked it didn't go all the way through, if that makes any sense.
I've always been able to get rid of the ghost alerts though. Sometimes, logging out and then logging back in works. Other times, if you refresh the Alerts page a few times, that number will go away. Switching from the full-site to the mobile site can also work.
I've always been able to get rid of the ghost alerts though. Sometimes, logging out and then logging back in works. Other times, if you refresh the Alerts page a few times, that number will go away. Switching from the full-site to the mobile site can also work.