It's mentioned in "Surely you're joking, Mr Feynman" or "What do you care what other people think"
But it was something like someone did an experiment to determine the spin of some particle. There was one data point that was way out, and actually was sufficient to dominate the result. Unfortunately it was the wrong result.
However many years it was, down the line, Dick looked at what he was doing and said "hey, if that particle was actually the opposite spin, everything would make sense". Then he went back to the original proof and said "hey, I remember thinking that proof was screwy, when it first came out!". Then he actually checked the maths, and said "ooopppsss".