When Charlie first asks Willa out to dinner, he is a corporal. Then at the restaurant, he is a sergeant, afterward he is a corporal again.
In the first two parts, Charles Main always went by “Charles” now, with no explanation, he prefers to be called “Charlie”.
George tells Constance that he and Orry wanted to put their bill together after the war and that it's not going to happen since Orry is dead, but they DID meet and DID put the bill at the very end of Book 2.
Baby Orry was born around summer 1864. At that time, Augusta Barkley (Charles Main's lover, mother of his son; Gus) wasn't even pregnant. Meanwhile here baby Orry is still a baby, while Gus is 5 years old.
Hope Hazard (daughter of George) was born in March 1861, but she looks like almost teenager here. George turns out to have a son, who isn't even that young.
Mont Royal wasn't destroyed THAT much couple months earlier (at the very end of Book 2).