This is quite a little but annoying thing. Get a like or a mention somewhere, go into your mentions menu, click on the notification link and it will open it, but the "1" won't go away - you will need to refresh the page one more time to see it disappearing, and I call that a bug because I am already reading the notification, the "1" should have disappeared already.
Duplicate of http://www.elkarte.net/community/index.php?topic=2321.0 (yes, it's the same thing.)
Missed it, I see it's 2 months old but still no fix or is it being worked on? :)
The mention failed xD Besides it's still a thing if you want to see it @Antechinus
(while you are reading this, note the "1" still present on top)
There are bugs that are older. :P
So is the missing-gallery-bug! :D