Olga Katsin-Miller writing as 'Sagittarius' has this to say about John H. Watson:
Holmes left one unsolved mystery,
The case of the strange M.D.
Was he ever qualified?
Had he anything to hide?
And why was he always free?
Facts of his previous history
Researchers fail to trace.
But there's something queer in his medical career,
For he never had a single case!
This may be poetic license, of course. Or it may not. We think we know from the canon that Watson treated Vincent Hatherley for a severed thumb, and a railway official for 'itch-mite' or scabies. But it's interesting that his credentials were being called into question over sixty years ago.