User blog comment:DaNASCAT/Technical Update: February 29, 2016/@comment-2112602-20160302084329/@comment-1251315-20160302085424

This bug has been around for a very long time. It is almost always caused when people hover over the notifications before the page has been fully loaded, making the Javascript that powers that section unavailable. Even when the page is fully loaded, the page stays in that state until a refresh. The fix is to refresh the page and don't look at notifications until the page has been fully loaded: I have only ever encountered this bug under that circumstance and leaving the page to load completely makes it work like it's supposed to.