We’d need more context. Something will have triggered that. For example your bug report where the register overlay loads the forum list - testing that will involve you being in inconsistent login states across two open tabs and that would trigger that.
It generally indicates you’re logged in or out on one tab and then either logged in as a different user or logged out or in on another.
If you can provide context as to what you were doing it will help, otherwise it’s likely just doing what it was designed to do.
I'm not entirely sure if I'm allowed to bump this thread, but I am having this issue too in a certain forum made with XenForo that I visit somewhat regularly.
It's quite inconsistent, I normally have a thread opened for a few minutes and inbetween I move from one tab to another on my browser (Mozilla Firefox v59.0.3), the same warning that AndyB shown pops up. Some other times, it shows up while I'm editing a post though.