The simpliest way for us to know if a note error occured is to wait until we get a g-state from the score follower. This would mean that somewhere in the score, the state corresponding to the expected note (n-state) has not been reached, i.e. we have something wrong in our alignment.
But knowing that we have a problem is not enough to say which kind of problem we have. To do this, we need to see what happens next: our way to know which kind of note error we had is to look at recognised notes indexes.
By checking pitches and indexes of notes following the one who was wrong, we will be able to say what was played: state succession will tell us what happened (see [OD01] and figure 4.1).