Suppose for a moment that the goal of teaching is to imbue your students with knowledge.
Teacher: one who gives knowledge to his or her students.
So would you be a failed teacher if.
a) What you say is not true.
b) Your student is not convinced.
c) You don't believe it (but its in the curriculum).
I know the above statements are vague. It is to stimulate conversation on the words used in this post itself.
Also, mention any other circumstances that might define a failed teacher.