In my field, by far the most significant is that of "Ottoman Decline." Everyone's heard about the Decline of the Ottoman Empire - the idea that the empire reached the peak of its power during the reign of Sultan Süleyman I (1520-1566), but that afterwards its institutions began to break down and the empire entered a period of stagnation and decay from which it was never able to recover all the way until its destruction in World War I. This "Ottoman Decline Thesis" started being challenged in the 70s and is now almost universally rejected. Nowadays book after book will tell you that the Ottoman Empire never experienced a period of decline at all, and that it was still a strong, innovative, and vigorous state long after the death of Süleyman the Magnificent.