The scripture says, "Be not righteous overmuch, and do not make yourself overwise; why should you destroy yourself?" (Ecclesiastes 7:16) Who can tell for certain what such a passage means?
For a teacher who is always defending the right cause, the right course or the right position, his constant sense of being in the right can mask his failures. The cause, course or position he's defending may be right but always being right (or thinking he is) can be addictive and it can blind him. There's something dangerous about not being challenged frequently and it's even more dangerous if we are never "proved" to be wrong—even when it's clear to just about everyone else that we are.