Beam Me Up, Scotty!: "Frankly, Scarlett, I don't give a damn." It's actually "Frankly, my dear, I don't give a damn."
Dawson Casting: Leslie Howard was 47, and felt he was far to old to play the twenty-something Ashley Wilkes. Justified because this film covers over ten years, and so he had to play the older Ashley as well. Also, Vivien Leigh was 26 years old when cast as the 16 year old Scarlett. Like the Howard example, this was justified as she continued to play Scarlett as the character aged.