In the final episode of season 3, Lisa Weil, as Carol Keene (aka. Carol Kaye), switches playing the electric bass in Shy Baldwin's band from left handed (all previous appearances) to right-handed.
Don Rickles is mentioned along with Bob Newhart. Rickles was years away from being famous in 1958, when the show takes place. Rickles and Newhart met in the mid 1960s.
Characters repeatedly refer to Cafe Wha? (famous as being the location where Jimi Hendrix was discovered) in Season 1, which takes place in 1958; however, Cafe Wha? opened in 1959.
However, per IMDb Guidelines: "We allow a good deal of leeway with antique equipment and machinery as long as it is "of the period" ... Linguistic anachronisms, too, are usually excused (modern words in historical films), and always
bear in mind that it's entertainment not documentary."
Although pronouncing the letter T in the word "often" is technically incorrect, this was not common usage until the 1990s.
Actually, according to the 1934 unabridged Webster's Second dictionary, "The pronunciation [of-ten], until recently generally considered as more or less illiterate, is not uncommon among the educated in some sections, and is often used in singing."
When Abe is sorting through children's albums looking for a song the computer can sing, he looks at one featuring Snoopy and Woodstock from the "Peanuts" strip on the cover. Woodstock didn't emerge as a distinct character until the 1960s and wasn't officially named until 1970.
All of the songs sung by Barbra Streisand are anachronisms because her first recording wasn't until 1963.
Following dinner at a Chinese restaurant, Astrid wonders aloud whether there actually is a General Tso. The menu item General Tso's Chicken was not introduced to New York City restaurants until the early 1970s.
The term "comedian" for women as well as men wasn't common usage until the 2000s. They would've called her a comedienne during the period during which the show takes place.
Many of the Checker taxicabs are post-1968. This can be identified because of the orange front and red rear side marker lights on the cabs when in profile.
In season 1, episode 4 while in the record store, Midge says she loves Bing Crosby, but after hearing Susie's opinion of him, says she will "never watch Bianco Natale (1954) again". In season 3, episode 1, however, when asked to join in a chorus of White Christmas at the end of the USO show, she claims not to know the song because she is Jewish.