It is astounding how many beginners on musical instruments are allowed to become careless, they themselves not realizing what it means or how much work will have to be undone and done over later in life. To me this negligence in the case of a beginner in music is the same as that of a child who when beginning the study of the multiplication table is permitted to guess of results, such as two times two equals six, or seven times six equals sixteen, and so on.Herbert L. Clarke, How I Became a Cornetist (orig. pub. 1934; reprinted in 2011 by BrassMusician.com), ch. 2.
The very first "guess" should be corrected and reasons explained; the child should be made to understand why twice two equals four. I classify all uncorrected errors as "microbes" which, although invisible to the naked eye, are deadly—even more deadly than an animal as big as an elephant.
One can run away or hide from or dodge an elephant, but not so with a microbe. These minute organisms multiply rapidly and in large number if not immediately drive out of the system.