When Jan and "Tex" are at the piano bar singing "Rolly-Polly", the man behind the pianist is strumming what is clearly a four-string electric bass guitar, but the sound is from a six-string electric guitar.
When Jan and Jonathon are talking in front of the interior design store about the car he is offering her, the same extras are seen multiple times. A woman with a blue coat and gray hat walks by four times, and a woman with a red coat walks by at least three times.
Jan Morrow is about to make a complaint to the phone company that she can never get a call through, because it is always occupied by the other half of her party line, Mr Brad Allen.
But when she goes to the elevator in her apartment block, (after an altercation with Mr Allen about this very matter) and meets drunk Alma coming out of it, one of her instructions to her maid is to call the office, and tell them she will be a little bit late.
A party line phone would not ring when any phone on the line was off-hook. To call the other phone on your line, you had to dial a special code and then hang up and wait for your phone to stop ringing, indicating that the other party had answered. This is not how Jan Morrow does it.
When Rock Hudson is split screened with Doris Day when she is in bed, you can see a fly buzzing around at the foot of it. A true high rise apartment in the city is unlikely to have even an occasional fly.