Good habits are learned and so are bad habits. If a mistakes are caught on early then they can be corrected before they reach the bad habit stage. Early on sparring and in training I believe a good coach should be able to point out the mistakes to the beginners and make them become aware of it themselves so that they can can avoid them. So even if you put two beginner boxers in the ring to spar you can caution them about what they're doing wrong and so forth so that the same mistakes are not repeated and therefore the fighter's are not hindered by any bad habits later on.

I think when it comes to the fundamentals the trainer must know it inside and out to be able to teach them properly. Most important thing a fighter can learn right from square one is posture/stance, which will give them balance and efficient movement, and from there the fighter has a good foundation for learning all the various movements.