"You won't fail me. You never did fail me."This line is easy to overlooking because Holmes says it to Watson in the midst of a lot of babble, much of it nonsense, while he is pretending to be delerious.
-- Sherlock Holmes, "The Adventure of the Dying Detective"
What makes the line intresting to us Watsonians (and what Sherlockian isn't also a Watsonian?) is that it's so true. Never mind that on several occasions Holmes dispatched Watson on some detective's errand which he failed to execute properly -- as he had several times before. One can hardly call the inability to be Sherlock Holmes a failure.
On a much deeper level than that, on the level of friendship and loyalty, Watson never did fail Holmes. He was always there when needed (poor Mrs. Watsons). And he put his life on the line for his friend almost every time Holmes asked him to bring along his revolver.
Watson is Holmes's rock and Holmes knows it. Victorian Englishman that he is, however, he would never come right out and say that -- unless, perhaps, he had the excuse of being delirious!