Noticing when a horse is making a failure is very important. Knowing when a movement is a failure or a success is the first essential visual skill to acquire. If a movement is made badly this can mean the entire benefit for that movement is lost completely – and it may even become damaging to the horse – and this applies no matter which method is used to train the movement in the first place.
It is never a good idea to expect perfection. If you expect perfection, you will be very likely to be dissapointed, but you must still know exactly what perfection looks like (and feels like) – and what it does not look like (or feel like) in order to work towards it.