Continuity mistake: In the shot when he turns the last 2 corners in the corridor before getting to the stairs, the file in Arthur's hands is facing the opposite way compared to before and after - the label should be on the inside, not the outside. (01:13:20)
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)
Suggested correction: She could have bribed her way into adopting a child. Someone who is desperate for attention could find ways to get what they want.
Suggested correction: It is not firmly established that Penny actually adopted Arthur - in fact, it's strongly hinted at that Thomas Wayne forced her into signing adoption papers in order to cover up Arthur's true parentage.
The established, as in recognized, backed up by documents, 'official' version the main character finds out and acts by, is the one contained in the report, newspaper clippings and flashback; son abused by the boyfriend of an adopted mother. Such story is impossible the way it is presented the moment we see details in a document that overblows it painting this 'adoptive' mother as single and with a history of drug abuse since 15 years old. Penny is not eligible to be an adoptive parent, and yet nobody seemed to have raised an eyebrow about that. If you want to assume that rather than being a mistake with overzealous details in a prop (check out of the original script of the movie, which has none of this ambiguity) whoever arranged the fake adoption documents kinda forgot to also make quietly disappear the mental and medical record invalidating their own fabrication, sure, do that! It's not exactly a small oversight - and really one would wonder why Wayne kept his bastard son with her at all.
Arthur is not Thomas Wayne's son. That was all in Penny's head.
Continuity mistake: When Arthur's neighbour walks on him drying himself up in her living room, the leftmost toy on the table in front of him has the shape next to the unicorn fitting the board in the front view, but in the reverse shot the piece is off the slot. (01:17:20)
Continuity mistake: Arthur is by his mother's hospital bed, smoking. Cut to a closer view ("That's the real me"), and his shoulders are in full sunlight, while he was in the shadow only a moment before. (01:20:30)
Revealing mistake: When we see Arthur go through taped Murray shows, he operates the VCR controls of his brand new Panasonic VCR. Problem is, it's not turned on. The red power LED on the far right is not lit, nor are any of the LEDs on top of the various keys (even the stop button had a static red light), or the large display where the clock would be. (01:22:15)
Continuity mistake: The two former coworkers ring Arthur Fleck's doorbell. The door is suddenly fully open when he lets them in, but was less than half opened when they were talking. (01:26:20)
Continuity mistake: Arthur leans against the door frame and has a creepy chat with Randall. Joaquin Phoenix's hair is plastered to his temple in a different way between shots. (01:26:30)
Continuity mistake: When Joker is dancing on the stairs he throws his cigarette. Then when the cops arrive at the top of the stairs we see him blowing smoke from the same cigarette. Then the cigarette is gone again. (01:30:40)
Suggested correction: They purposely stuck the images in a way that it repeats itself over and over again. It's intentionally made in a way of not flowing straightforward.
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)
Audio problem: In a bit of movie magic, nobody touches the TV set in the dressing room, but it adjusts its volume perfectly by itself, to the point that not only does not disturb the conversation with Murray and the producer, but even when Arthur is all on his own and sits back in his chair the TV, still turned on and showing a program, is perfectly silent. (01:35:10)
Continuity mistake: Murray is leaving the dressing room assigned to Arthur, but he asks "one small thing." He is holding the cup with the hand over the lid; when he leans back into the room he is holding the cup by the side. (01:36:20)
Continuity mistake: In front of the audience, Joker sits down. After awkward silence he says that it's all just as he imagined and Murray Franklin has a comeback joke. The guest to the far end of the couch has his arms crossed, which become just his fingers entwined at the cut. (01:39:20)
Continuity mistake: During the first part of the interview, Murray Franklin's hands keep changing position inconsistently holding cue cards or being flat on the desk. (01:39:25)
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)
Continuity mistake: Leaning against the car during the riots, Joker's face paint is smeared with a tear exactly the way it was when he walked out of the subway before the Murray Franklin show. But of course it went through a lot of fixing since. (01:46:45)
Continuity mistake: When Joker is in the police car, the left eye paint flows across the corner of his mouth. Then the car crashes, people pull him out of the car, and the paint just reaches his nose. Then he stands up and the previous style is back. (01:46:50 - 01:50:10)
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)
Continuity mistake: Between the shot when Joker widens his smile painting it with blood and the following wider angle, the items people in clown mob are waving around change suddenly. (01:52:10)
Factual error: Based on the films being shown at the theater, the movie appears to be set in 1981. One of the TV commercials shows the Energizer Bunny, which didn't make its first appearance until 1988.
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.
