This isn't a bug so much as the Websockets-powered friend sidebar being faster than the main page of the user, which reads from cached data (which can be slower).
This is an issue with caching, as Nope stated. You'll need to find a shortcut for your preferred browser that will reload the cache for the website. For Firefox it would be Ctrl+Shift+R or Command+Shift+R.
This is related to the 'friends bug'.
@ScruffyRules: What's the friends bug?
@ScruffyRules: no, it's just caching issue. Sidebar has no cache buster so it always will show whatever is cached previously. Disabling cache will update sidebar properly.
@Nope: +1 for correct info. tested and confirmed its a local caching issue.
Just had this after the friends fix so yeah, not that.