What Scientific Beliefs Have Been Flat Wrong

Father of Behavioral Economics Richard Thaler asks Edge contributors what long-held scientific beliefs were later proved to be flat wrong. He follows this by asking why they were held for so long in the first place.
While this is an interesting question in itself from a historic perspective, to me it is a lot more profound. The history of science teaches us one thing above all else, scientists are almost always wrong. Every theory that we have had since we learned to make theories has been proven incomplete. The entire edifice of science is based on the idea of continued, incremental improvement and monumental, continuous failure.
Since we have been wrong before (and for fantastic reasons that don’t take anything away from science) the real question is, which of our pet theories today will be proved to be spectacularly wrong down the road?
The flat earth and geocentric world are examples of wrong scientific beliefs that were held for long periods. Can you name your favorite example and for extra credit why it was believed to be true?
Read Wrong Scientific Beliefs (Via Edge) (Images)