When the salesman and Betty are on the stairs and he is leaving, he puts his pen in his shirt pocket but when he turns around he has it in his right hand.
The stamps on the package delivered to Don's office are 8 cent stamps picturing General John J. Pershing. Those stamps weren't issued until 17 November 1961, so they couldn't have been on a package mailed in 1960.
"Agua de Beber" ("Water to Drink") by Astrud Gilberto plays on the soundtrack during Betty's fantasy sequence, at the end of which it sounds like it's actually playing on the radio. This episode was set in October 1960; "Agua de Beber" wasn't released until 1965, on The Astrud Gilberto Album.
The version of "Fly Me to the Moon," sung over the credits, is by Julie London. This wasn't released until 1963, on her album "The End of the World," yet series one is set in 1960.
Carl Winter, Peggy's date at La Trombetta, says, "My sister's a secretary for Bulova in Flushing." Bulova has never had any offices or factories in Flushing, NY. The original headquarters, from 1953 to 1982, was in Jackson Heights, Queens, at 72-20 Astoria Blvd. It was a distinctive Art Deco structure at the interchange between the Grand Central Parkway and the Brooklyn-Queens Expressway. The company moved its headquarters to Woodside, Queens, in 1982.