Which theory proposed by Galileo is considered a "big mistake of his life"? What did the theory state, and why was it a "big mistake"?

The erroneous theory of Galileo was that tides were a result of the earth's what he called the inconsistent revolution around the sun. He was right about the most important thing of the earth revolving around the sun but didn't realize that the moon (and sun) exert a gravitational pull on the earth's oceans and cause the diurnal tides to occur.