Fire and Knowledge

It ends up Sherlock Holmes never actually said “Elementary, my dear Watson.” Here’s what Chip & Dan Heath say about it in Made to Stick:

One of the most famous misquotations of all time is attributed to the fictional detective Sherlock Holmes. Holmes never said, “Elementary, my dear Watson.” This seems hard to believe—the quote is perfectly suited to our schema of Holmes. In fact, if you asked someone to name one Sherlock Holmes quote, this would be it. His most famous quote is the one he never said.

Why did this nonexistent quote stick? It’s not hard to imagine what must of happened. Holmes frequently said, “My dear Watson,” and he often said, “Elementary.” A natural mistake, for someone inclined to quote from a Holmes mystery, would be to combine the two. And, like an adaptive biological mutation, the newly combined quote was such an improvement that it couldn’t help but spread. This four-word quotation, after all, contains the essence of Holmes: the brilliant detective never too busy to condescend to his faithful sidekick. (p. 239)

(Wikipedia has some other famous misquotations.)