explain somewhere that in some cases the signing key may have to be updated
In the documentation to download and verify the ISO with GnuPG (http://tails.boum.org/install/download/openpgp), it is said that
If you are doing the verification for the first time, download the Tails signing key and import it in your keyring. If you are working from Tails, the signing key is already included.
If a user have downloaded the signing key a long time ago, or uses an old Tails release, it may not be up to date, and therefore the verification ends up with a "this key is expired" error. A lot of users keep using this method to verify the ISO and send us bug reports or emails, worried about our signing key not being valid anymore.
I think I'll add an entry to the FAQ about this, but I doubt it will avoid all these users from sending bug reports... If the documentation suggested the users to update the signing key before verifying the ISO, it would solve the issue.