Why did Sherlock Holmes ask Watson to whisper "Norbury" in his ear in The Adventure of the Yellow Face?

At the end of the story, Sherlock says to Watson that if he ever gets too confident with a case or too complacent, that Watson should whisper the word "Norbury" in his ear. Sherlock says that he will be "infinitely obliged" to Watson for remembering to do this.
Earlier in the story, Sherlock and Watson are visited by a client who tells them about an empty and mysterious house in a place called Norbury. The client explains that his wife has been visiting this house in the middle of the night, and that after following her one night, he saw a hideous yellow face in one of the windows. Sherlock confidently predicts that the client's wife is being blackmailed by her ex-husband, who, he thinks, is falsely thought to be dead.
However, when they visit the house in Norbury, Sherlock discovers that the truth is quite different. The yellow mask is worn by the wife's black child, born of her relationship with her ex-husband. Because of racist attitudes at the time, the wife feels that she must hide the child away from the sight of her neighbors.
Thus, when Sherlock asks Watson to whisper "Norbury" in his ear whenever he becomes, in future, too confident with a prediction, he is asking Watson to remind him of this case. He wants to remember his complacency in this case so as not to repeat it in a future case.