Confluence shows the following error when the session expires during page editing/creation:

Your request could not be processed because a required security token was not present in the request. You may need to re-submit the form or reload the page.

Sometimes, the edits that have been done are lost and cannot be recovered once users are logged in.

Therefore, Confluence should warn a user when a session is about to expire, so that the user can save the contents first before it expires