Factual error: In the beginning of the movie, while dancing with the sign, the guitars in the window of the music store are D'Angelico guitar designs that weren't released until the 2010s. (00:01:35)
Factual error: The cart behind Arthur during his dance for the kids has drawers of different colors. That is color coding reflecting the Broselow Tape, a tool used to measure pediatric patients and give estimates of the appropriate scaling for treatments. It's a system that started only in 1985, while the movie takes place in 1981. (00:28:05)
Factual error: In the elevator, Arthur meets the single mom living on the same floor, Sophie Dumond. In Sophie's shopping bag, the one recognizable food item is a box of Betty Crocker's Hamburger Helper - Potato Stroganoff. Thing is, the packaging uses (look at the design of the 'menu idea') a design predating the introduction of the Helping Hand mascot, which happened in the late 70s. She just bought it, it can't be ancient, long expired food. (00:19:35)
Factual error: When Joker is following his neighbor to her work, the 'no standing' sign is not from the time period. Also when Joker is fleeing from two police officers into the subway at 18 avenue, the subway sign with the red EXIT background is not of the time period.
Factual error: The ambulance that crashes into the police car Joker is riding in towards the end of movie has the styling of a contemporary vehicle, not one from the early 1980s.
Factual error: When Joaquin Phoenix and Zazie Beetz are taking a walk after Arthur's performance, between the arcade and the newsstand there's a modern video intercom with keypad, not quite fitting the 1981 setting, since the first model of its kind was introduced in 1984 (kinda odd to leave something like this in when they went through the trouble of placing appropriate arcade posters really close by). (00:45:25)
Factual error: Following Zazie Beetz, Arthur arrives in front of the bank. The crossing is using red colored tactile paving. While technically already invented, truncated domes paving was not adopted in the US in the early 80s, but began appearing in the early 1990s at public transportation stations, and it was not until 2001 that they were used in curb cuts. (00:24:35)
Factual error: The Waynes went to a theater with 5 movies, with the main spot being Blow Out and Zorro the Gay Blade, who came out on July 24th and 17th of the year. You can also see marquees for other July movies; Wolfen, on the 24th, Arthur, on the 17th. But the narration that opens the movie said it was Thursday October 15, and the events of the movie span across at the very least two weeks (the Joker gets booked on Murray's for "next Thursday" when at least a week has passed since the intro). So the theater is showing in November movies that are over 3 months old (7 for 'Excalibur'. Last movie to be featured) and no new releases. (01:49:10)
Factual error: In the so called homage to "Network", the second TV screen from the left in the top row broadcasts a TV PSA with Clint Eastwood ("The thrill can kill") that is from the Reagan era, years after the events of the movie. Specifically from 1987, which is also the year of the Kellogg's ad in the TV under it, with Keanu Reeves. Another ad playing in the bottom right corner is a Rolling Rock beer ad that is from 1986. (01:46:25)
Factual error: Heading home after the first encounter with the social worker and the incident with the kid and the irritable mom, Arthur is trudging to get his meds at Helms' Pharmacy. Parked in front of the pharmacy there's a sedan, a Mercury Grand Marquis, which by bumper design is a post 1988 model, way past the events of the movie, in 1981. (00:09:20)
Factual error: It is established that Penny Fleck adopted Arthur and that he's been abused. In her file, when Arthur reads it, you can see that she was admitted the first time to the psychiatric hospital at 15 years of age, had multiple episodes with drug abuse, and the file mentions she is 25 and single on the date of the report, 11-2-1952. A single parent already had rather slim chances to adopt in the 50s, but a known mental patient and drug abuser, not a chance. (01:13:40)
Factual error: The subway in the movie has been recreated with exceptional attention to details, including custom Gotham Transport Authority logos and even maps of the fictitious subway. Throughout the movie in general, references to the actual NYC have been carefully removed. However, before he gets to the station just after the iconic stairway dance, Joker runs in front of a store labeled Meat Market that has a 718-992 phone number, from Bronx. (01:31:50)