Today I experienced quite a strange behavior regarding the site login.
After being active (and logged in) on several sites for about an hour I stumbled about a meta post on a page I hadn't a user on (Monero) as one does. To up vote said post I joined the community with my regular user. Then I went over to have a look at their main page where this weird top bar greeted me.
There's my user avatar, my association bonus reputation which I had clicked on just before taking this screenshot and this weird notification in the achievements tab. I did not logout on any tab. In fact I was chatting in another tab and using the Internet of Things site without impact. Only when I went here to meta (which I didn't have open in any tab) I had to completely login again.
It happened on Chrome Version 63.0.3239.84 on Windows 10.