The other day I was thinking about the process of training a horse - namely, first he is treated like he does not know anything and you, the horseman, are “black and white” and nothing in between. In other words, the horse needs to be shown clearly what the correct response is and nothing else. Then, as time goes on and the horse “gets it”, you can become grey or “sloppy” and the horse will still do his job by correctly guessing what it is that you wanted. If he does not guess right and you have to correct him, it will not be a big deal for you or him.
What is the biggest mistake people make in training the horse? They start out grey - one time a mistake is accepted, another time it is not. This is what a lot of books and people in the business refer to as “inconsistency”. It is also the fastest way to make for a confused animal that wil react to such treatment in various ways.
What drives most people to make this mistake? Lack of knowledge is probably the biggest reason. But, sometimes, you also find the person who just doesn't see that they are making the mistake - in private life they are the “easy going” person for whom “anything goes”. In this kind of disposition, “grey” is the modus operandi for most things in such a person's life and training the horse in this way is just another part of life (in their minds they are “consistent” with their life philosophy). Finally, there is also the person that desperately wants to have “light” or “soft” hands and is therefor impeded from raising the pressure enough on a horse to clearly show him the “black” and “white” sides of the thing they are teaching. Such a person is afraid of ruining the horse so much that they are impeded in conveying the knowledge.