Can it happen that a student has the right idea, but then doubts himself and unlearns it, because it was improperly taught?
In some respect I actually don't believe in "teaching" -- I think there is only presentation and learning. A teacher is basically there to present information, to plant seeds, and help/support the germination and sprouting process as the student learns. The student, however, is the host of the presented information, and that says a lot ! I wonder what are the bees that carry a pollen from one person to another ? There's actually something magic there that is not truly within the control of either teacher nor student.
But in any case, once something is learned, it is learned (the idea has matured). However, during the germination and sprouting process, doubt can enter in and if these stages were improperly supported, then yes, the right idea can go dormant... until the right circumstances find it again and bring it back to life.
Can it happen that a student's instinct leads to the right place, he verbalizes it wrong, or doesn't understand the teacher, and then gets caught in an internal conflict by trying to force himself what is wrong - becuase now he believes this wrong is right?
Yes, sure. But in this case, the right thing is not learned yet -- however, there are a number of things that can happen in that scenario. A seed may be planted, it may even germinate, but it finds rocky soil just above and the sprouting process is delayed. I believe that if something has truly been planted, has truly germinated, it will eventually force its way through -- if not one way, then through another. So, eventually the inner conflict, the rocky soil, it will become too great for the idea to stay in the same place. Sometimes this means that a student changes teachers, sometimes it means they go out on their own ... it could mean a lot of things.
But, it is these planted ideas, trying to take form, trying to sprout, that may lead a student into other grounds. In this case, even if the concept was poorly presented (and poorly supported) -- the lesson will eventually be properly learned. If the idea's host is in earnest and seeks to find answers, well, that is some kind of search for food and water. The host will probably walk through deserts to find it, and during that process, that which was unhelpful, or improperly presented, it will fall by the wayside -- because, let's face it, it's quite a journey to be lugging around bags of garbage !
Ideas can be poorly presented, but the fabric that makes the idea valuable will survive and may even be carried by an invisible, magic bee to the learner, despite the poor presentation.