What actually happened: A user’s session was destroyed (i.e., they logged out, or the session expired) while the same user made a request with the same session key.
Why it happened: For example, it could happen if the user had two tabs open, and logged out in one of the tabs, while a request was also made from another tab. If both happened in quick succession then one would hit this error.
Do you need to worry about it?: Not unless you see lots of events like this in the logs, in which case there is something wrong. If you found the error just once, then it’s nothing to worry about.