Picking the Wrong Face
If you pick the wrong face during the "practice" stage of the enrollment process, you will be informed immediately and prompted to try again.
This does not happen during Log on (or trial log on). You will not be alerted until you have picked all of your faces. At that point, you will be asked to start log on again. Alerting you in advance of moving to the next face makes it very easy for anyone to guess your Passfaces.