Yubico ykneo-openpgp before 1.0.10 has a typo in which an invalid PIN can be used. When first powered up, a signature will be issued even though the PIN has not been validated.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2015-3344
https://developers.yubico.com/ykneo-openpgp/SecurityAdvisory%202015-04-14.html