Along those same lines, the internet itself can be an "Armchair Instructor". I was teaching a student in private lessons, and he took some initiative to look up the next belt's form and start memorizing it. I am ok with that. But, when it came to the performance of the techniques, he was performing them differently than I had taught him, because "That's how they did it on YouTube". Now, it wasn't necessarily wrong, but it wasn't the way I teach that it should be done. Fortunately, so far, all my students have been adults in private lessons, so I haven't had to interact with the other Armchair Instructors talked about here. I am laying the groundwork for opening up a school here, though, so I am interested in topics such as these.
