When Don and Betty are speaking in the bedroom, as Betty says the line "How can he talk that way to me in the condition I'm in", Betty's left hand is raised holding a cigarette. In the next shot her hand is resting on the bed.
After seeing Ann-Margret sing Bye Bye Birdie on film, Sal Romano remarks that he'd seen Susan Watson do "it" (presumably meaning same number) on Broadway but opines that she couldn't begin to measure up to Ann-Margret; in reality, the title tune was written especially for movie and was never performed by Watson on stage.
When Don confronts his brother-in-law about concerning Betty's father and how to get home with no vehicle, Don tells him to take the New York Central and the "Broadway Limited from Penn Station." The "Broadway Limited" was an all private room reserved train to Chicago. It did not accept coach passengers for intermediate stops.