Don and Megan are at a luau, and then later that night we see her in her bra and panties. We know it's the same night because she talks about the people who watched her soap and wanted her autograph. But the dress she was wearing at the luau was backless-no bra.
Incorrectly regarded as a goof. Actually a flashback: When Don and Megan return from Hawaii, Megan asks the doorman how he is while still wearing her hat and coat. When the door man immediately collapses, she runs over, suddenly completely free of all her winter gear.
Peggy's advertising firm talks about problems with a "Super Bowl" advertisement. This would have been broadcast during the 1968 game. But the game wasn't called the Super Bowl until the 3rd game, in January 1969.
The young soldier Don meets in the bar introduces himself as a PFC. But when we see him at the wedding ceremony on the beach, his dress uniform has the "mosquito wings" stripes signifying the Vietnam era rank of an E2 , one below the E3 PFC. [At the time of the wedding in December, 1967, the insignia was correct for a PFC E-3. The rocker stripe was not added to the PFC insignia until May 28, 1968, at which time the single stripe was assigned to Private E-2.]