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I haven't really experienced such before, the only message I get sometime is when I log into a new browser and forget to hit the stay logged in button. So I get to view the forum as a guest until I login again.

Apart from that, you can also find similar messages when you enter from two different devices in same time and then you log out one of them. Automatically, your account will be logged out of all devices. You can try.
@OP, I think the error message that you get is when you are on the middle of creating a thread while your account get logout due to inactivity. This happened to me many times because I’m very slow to create a thread.
I think your point is about running out of active sessions, temporarily you won't be automatically logged out unless you uncheck always log in and log out from one of two different devices.