No biggie. It hasn't happened to me in a very long time, but just now I had my main forum menu go berserk with large numbers of unread posts to read in every forum.
Logging out and logging in made it revert back to the actual number of posts I have not yet read today.
I can recreate the same issue probably 100% of the time by starting a Reply to a post, going away from the Reply's tab/window in my browser for a while, then coming back and finishing the reply. I don't know for sure what the time frame is for how long the Reply window must be open to create the issue, seems something like maybe 30 minutes or so.
It's always done that BTW. I believe I checked it with both Safari and Firefox. It's probably a cookie issue. Like you said no big deal though.
I don't know if you can delete the whole post but there is some kind of time limit on editing. If you double post or want it moved or something PM me, I can take care of it, no time limit for me on editing, deleting or moving.
Although if you just decide you shouldn't have said something, think twice next time.