Early in the movie, boxes of glassware in the back of the club have large modern barcodes. An hour into the movie the boxes are shown again, with the barcodes taped over.
In the café, Alice's heart pendant changes its orientation several times between shots.
As the two girls arrive in a taxi and are allowed to enter the disco, they change places between shots after the doorman lets them in.
A table cloth rotates through 90 degrees between shots at Rex's Bar.
In Rex's bar, the ashtray starts off in the center of the table, and then gets picked up moved to the center of the table by Charlotte.
When the nightclub manager is in the DJ box and says, "Michael, "Good Times"", meaning play the song "Good Times" by Chic, the record cover that the DJ picks up shows an image of a white, blonde female artist, clearly not Chic. Also the visible label on the actual record is blue, not the red/black or red /green Atlantic Records logo, Chic's record company label.
The "past perfect" that Bernie picks up on in a dialogue with Des, is actually just the "simple past" tense. The past perfect would not have been "I was approached", but "I had been approached". (A serious error for the self consciously erudite Whit Stillman.)
Several of the main characters call themselves "yuppies", and one reports seeing graffiti reading "Die, Yuppie Scum!", even though the film was probably set in 1980-81. While the first known use of the word "yuppie" was in 1980, it did not gain common use, let alone negative connotations, until 1983.
In scenes where the main characters are seen riding the subway, the subway cars are immaculately clean and graffiti free. During the early 1980s (the time this film takes place), subway cars were notoriously gritty looking, with graffiti covering the inside and outside of the cars. Police officers were also a common sight, as crime on the subway was at an all time high. During the late 1980s, the Metropolitan Transportation Authority (MTA), which oversaw the city's subway services, purchased a newer model from Canadian manufacturer Bombardier, that was made of graffiti-proof alloys and had a different seat layout from previous trains. This model is the one seen in the film.
In the background of the street scenes, you can see car models made in the 90's, not the 80's.
The disco riot at Comiskey Park (the infamous "Disco Demolition Night") was actually in 1979.
In one of the office scenes.
As Alice and Charlotte walk down the hallway at the publishing company.
In the penultimate scene, Josh and Dan exit into a subway station with posts topped by red globes. Subway portals with red globes are exit only. They never could have entered there. As we see Des and Charlotte walk away, we see an entrance for the same subway station (World Trade Center) marked with green globes. The green globes designate 24-hour entry. There Josh and Dan could have actually entered for the subway.