If Alice has already certified Bob's key, but she later decides that she wants to adjust the certification (e.g. with a notation, by adding an expiration, or by moving from an exportable to a non-exportable certification), she cannot do that with gpg --quick-sign-key, because it reports something like:
"Bob <firstname.lastname@example.org>" was already signed by key 1234567890123456
I note that if the existing certification is a non-exportable certification, and the new certification *is* exportable, then gpg --quick-sign-key will actually make the new certification, and drop the old one. That's good! but it doesn't help if the user wants to go the other direction (exportable to non-exportable), or if they want to add a notation with --cert-notation
I also see no way to set an certification expiration using --quick-sign-key at all, but perhaps that's a different issue.