It comes as the last line of "The Yellow Face," that Canonical tale that made a beautiful statement of racial acceptance in 1893. Sherlock Holmes say this:
"Watson, if it should ever strike you that I am getting a little over-confident in my powers, or giving less pains to a case than it deserves, kindly whisper 'Norbury' in my ear, and I shall be infinitely obliged to you."
It is a very special moment amid the many cases of Sherlock Holmes. He has failed before, against Irene Adler or Captain Calhoun, but even in those matters his theories weren't at fault . . . both foes just moved too quickly for him. In, "The Yellow Face," Sherlock's solution to what's really going on in the cottage on the other side of the field from Grant Munro's villa is wrong. There are no Scotland Yard men on scene to see this rarity. No one loses life or limb over it. But Sherlock Holmes is wrong, wrong enough that he hope Watson will remind him of the moment as a caution.
Sherlock Holmes gives Watson that investigation "safe word." It is his to give.
John Watson doesn't pull it out himself and then beat Sherlock around the head with it.
"Remember Norbury, Holmes? Remember, NOR-BUR-Y!?!" That isn't how "Norbury" comes about. It's Holmes making sure he has a witness who understands what that failure meant to him, and setting himself a boundary, and a goal.
Sherlock Holmes has learned his lesson, and we never hear of Watson needing to use that word in the years that follow. One could theorize that the good doctor was kind enough not to write of that later moment when he used a whispered "Norbury" to his friend. But I like to think that Sherlock Holmes didn't need it.
We all must acknowledge our personal Norburys. We have to see those mistakes for ourselves, as Holmes did in "The Yellow Face." And if we learn that lesson well enough to share that acknowledgement with our closest friend, so much the better. There are better words for pointing out the mistakes that others may have made. Lord knows, Sherlock Holmes used a few of those.
"Norbury," however, remains something we must give a friend from our own hearts, and our own lessons, just as our favorite detective first did in print in 1893.