An attempt to save an article in the visual editor after a period of inactivity results in the "Invalid token" error message and saving fails. At this point, the user is completely stuck and their entire work is lost.
Loss of session data can also occur in the regular source code edit window, but is easily rectified by repeated saving; this doesn't work in the visual editor.
I would classify this as user-hostile behavior.
Version: unspecified
Severity: major
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=50402