YubiCloud demo Touch the YubiKey button:

                   OK  The OTP is valid.
              BAD_OTP  The OTP is invalid format.
         REPLAYED_OTP  The OTP has already been seen by the service.
        BAD_SIGNATURE  The HMAC signature verification failed.
    MISSING_PARAMETER  The request lacks a parameter.
       NO_SUCH_CLIENT  The request id does not exist.
OPERATION_NOT_ALLOWED  The request id is not allowed to verify OTPs.
        BACKEND_ERROR  Unexpected error in Yubico servers. Please contact them if you see this error.
   NOT_ENOUGH_ANSWERS  Server could not get requested number of syncs during before timeout.
     REPLAYED_REQUEST  Server has seen the OTP/Nonce combination before.
            BAD_NONCE  Answer Nonce is different from the request Nonce.
     CONNECTION_ERROR  Impossible to make a connection with the YubiCloud servers.
     OTP_IS_DIFFERENT  Answer OTP is different from request OTP.
   OUT_OF_TIME_WINDOW  Timestamp difference with the Yubico servers is bigger than yubicloud_max_time_window.
       SERVER_TIMEOUT  Timeout while waiting an answer from the server.