The BFI book on Blue Velvet quotes an interviewer who mentions to Lynch in passing that Kafka said he is a great writer because he can dream while awake. When I read that it completely opened up the poem "Sleepers Awake" by John Ashbery -- which in turn gave me the idea for my doctoral thesis in which the discussion of Sleepers Awake took up chapter 6. Three years later it turned out that Kafka never said that, as far as I could discover. I had enough evidence in other areas that the basic point still stood, but this mistake gave me my whole trajectory at Oxford.
A few days ago I put up a post of quotations by Ruskin while working on my budget, but could not find the quote I was looking for in the first place. Today, I discovered that it looks like Ruskin never said one of the main quotations I was searching for:
"There is hardly anything in the world that someone cannot make a little worse and sell a little cheaper, and the people who consider price alone are that man's lawful prey."
There is an Oscar Wilde point in here somewhere about the value of error, but I am sure you can all see it for yourself.