Doyle uttered a couple of inaccuracies in The Sign of Four. These are generally explained, by critics and Doyle himself, as the oversights of rapidity of thought and execution and a consequent inattentiveness to detail at odd moments. Doyle admits in his Memories and Adventures that his readers would point out the lapses in memory for details that occurred in his various books.
The boldest inaccuracy is that in A Study in Scarlet (1887) Watson's war wound is in the shoulder, "at the fatal battle of Maiwand. There I was struck on the shoulder by a Jezail bullet, ...," whereas in The Sign of Four (1890), a later work, his war injury is in the leg:
I had observed that a small vanity underlay my companion's quiet and didactic manner. I made no remark, however, but sat nursing my wounded leg. I had a Jezail bullet through it some time before, and, though it did not prevent me from walking, it ached wearily at every change of the weather.
Another inaccuracy relates to Sherlock Holmes' infamous addiction, remembering that at that point in history, drugs had not reached the level of harm and destruction at which they reside today. Holmes use of cocaine had been established, yet in The Sign of Four, Watson surprises us with asking if Holmes is using one of two drugs:
Yet upon that afternoon, whether it was the Beaune which I had taken with my lunch, or the additional exasperation produced by the extreme deliberation of his manner, I suddenly felt that I could hold out no longer.
"Which is it to-day?" I asked,—"morphine or cocaine?"
He raised his eyes languidly from the old black-letter volume which he had opened. ....