What famous scientist mistakenly thought he had discovered plutonium -- a decade before it was actually discovered?

TechTalk radio ep. 251 includes a host of Thanksgiving-esque tech tidbits and, sadly, a trivia question that perhaps on I find interesting: What famous scientist mistakenly thought he had discovered plutonium -- a decade before it was actually discovered?

If you're the kind of geek that gets off on the historical minutia surrounding elemental physics -- and you know who you are -- you'll also likely enjoy pointing out all the errors I make in my weekly radio factoid from last fall.