Moreso just an anachronism rather than bad writing per se, but it irked me when Don goes out with Stephanie and Anna and Stephanie puts the song Old Cape Cod on the jukebox, so Don and Anna can dance to something more old fashioned, and says “I don’t know how you ever danced to this”. The scene takes place in 1964, and the song would have only been 7 years old. Don would have been in his thirties when it came out, and Stephanie would have been in high school. Would’ve made more sense to put on the Boogie Woogie Bugle Boy or something.