Megan uses a Kodak Instamatic to take Hawaii pictures [beach wedding]. This is a square format camera. Later, vacation slides are projected as conventional rectangular images.
When the Drapers are at the New Years Eve party with their neighbors, Megan mentions that they missed midnight. A while later when the doctor gets called to the hospital on an emergency, Megan says he won't get a cab, because it's Sunday morning. January 1, 1968 was actually a Monday.
Peggy's boss Ted, speaking of his time away says, "It was sort of a retreat for my wife and I." The hyper-correction of saying things like "just between you and I" or "for my wife and I" would not have been common in the 1960's, and only became regularly heard in the late 1980's. Someone of Ted's education in the 1960's would have used the grammatically correct formula "for my wife and me."