In the story "Valley of Fear" Holmes asks Watson the following:

"You have heard me speak of Professor Moriarty?"

Watson replies:

"The famous scientific criminal, as famous among crooks as --"

In "The Final Problem" Holmes asks Watson:

"You have probably never heard of Professor Moriarty?" said he.

Watson Replies "Never."

Anybody see the error here?

Editor's Note: I see the error; it's two different books.