Possibly include something about the user changing their browser settings to check for an updated version of a page more frequently?
FAQ 161 - "Why do I receive an "invalid cookies" error, and how can I fix it?"
Possibly include something here about the user performing a hard reboot of the system?
(This may actually belong better in howto_userdoc, however, I'm going to propose it here first. If it is more appropriate there, then I will move the discussion there, but I didn't want to see it being changed in the FAQ *and* the tutorials. I don't know of a compelling need to have the users hard rebooting their system twice, and you know someone is going to do it if we change it in both places!)