I'm having an odd and inconsistent problem where certain inbox notifications are re-appearing within my inbox every few days, even though the date/time for the notification is a week, two weeks, a month, two months or even three months ago. Unfortunately my screenshot didn't take so I don't have a screenshot at this time. The messages I'm getting within the inbox that are repeating though are posts that have been migrated, moderator elections, and other non-answer related items.
Usually a page refresh or clicking on the inbox notification badge, opening the inbox dropdown, and clicking outside of the dropdown clears the messages out of the timeline. Yet every week or so they all re-appear again.
Update I finally was able to capture a screenshot. This was taken this morning at 8:06am. Even though I have an inbox notification of "7", you can see that the "newest" inbox notification dates from May 22nd and the other 6 "new" notifications are from as far back as Feb 2014.