למד לשונך לומר איני יודע

Teach your tongue to say, "I don't know."   

למד לשונך לומר איני יודע 

(Berachot 4a)

In one of my teacher training classes the rrominent instructor said that you have to know your topic cold. Someone cited this quote and asked, "Isn't it noble to say you don't know?" The principal, who was teaching the course, said that it's a good thing now and then but if you're saying it every day and you're a teacher something's not right.