In some of the stories about Sherlock Holmes, Dr John H. Watson is referred to as James Watson. The reason for this is unknown, but one theory is that the good doctor used the name James when enrolled in the military.

This is not the only discrepancy however; it seems the wound that ended Watson's military career has a tendency to move around. In A Study in Scarlet he mentions that he was hit in the shoulder, while in The Sign of Four he is experiencing pain from his war wound in one of his legs. In The Adventure of the Noble Bachelor a wound is mentioned in one of the limbs, but we don't know which.

This is of course cirumstantial evidence, but it seems that we are not told the whole truth about Watson. Maybe he has an evil twin who sometimes manages to upstage him?