Let's see... over the course of Harry's time at Hogwarts, we had:
1) A professor who was an escaped convict in disguise.
2) A government worker who literally scarred her students with her punishments.
3) A completely unqualified teacher who REMOVED A STUDENT'S BONES FROM HIS BODY, GOD DAMN.
4) A professor who had the Dark Lord himself on the back of his head, hidden only by a turban. NBD.
Any of these situations could have been avoided with a simple background check, or like, paying any attention at all.