Continuity mistake: When we first see Tara, as well as in the shot where Scarlett is running away from the Tarleton Twins to meet her father returning from Twelve Oaks, there is a very large tree directly in front of the main house. There are two white peacocks underneath it. In all subsequent shots of the house, when Scarlett and her father are talking in the sunset and again when her mother is arriving from taking care of the poor white trash Slattery family, the tree is completely gone.
Visible crew/equipment: When Mammy catches Scarlett trying on a pretty bonnet when she is supposed to be in mourning, when the camera pans wide to show the room, in the mirror on the wall behind Scarlett you can see the shadow of the grip holding the mic above Scarlett.
Revealing mistake: After Frank Kennedy's death, Scarlett is drinking in her bedroom, when she spies Rhett arriving outside her window. She then races to the mirror to brush her hair. She does not actually touch her hair with the brush strokes, probably so as not to disturb her hair, which had been specially styled by hair and makeup staff.
Answer: If she looked rich she could trick him into thinking she wasn't marrying him for his money.