It's not the student's responsibility to protect the teacher's ego, it's the student's responsibility to be reasonable. Standing up and yelling because the teacher forgot a semicolon in some example code, for instance, is a waste of everyone's time, not just the teacher's.
Your somewhat extreme example notwithstanding, are you saying it's unreasonable for a student to point it out when a teacher is giving incorrect information to the class?
I'm saying "It depends." My experience with that behavior indicates that it's usually counterproductive- either nitpicking like my previous example, or axe grinding that isn't really helping. Think an Ayn Rand follower interrupting a class on Marxism every few moments.
The problem, really, is that it's impossible to make a fully qualified statement and still be engaging. As a speaker, you have to skip over some detail somewhere, thus there is always going to be a place to jump in and act an ass.
