Continuity mistake: Arthur is taking notes during the standup comedian's performance. There are a cigarette lighter and a packet next to him on the table; in the close-up that follows, they have changed position. (00:25:45)
Continuity mistake: After the gig at the comedy club, Arthur and the neighbour Sophie pass in front of a newsstand. The copy of the Examiner saying "Killer Clown On The Loose" is kept in place by a clothing pin placed diagonally, top right-to-bottom left. In the close-up when Arthur looks at it, the pin is top left-to-bottom right. (00:45:25)
Continuity mistake: When Joker is beaten up at the beginning of the movie, he changes position on the ground between shots. The pieces of the sign next to him also change position; in the last shot that lingers till the movie title pops up, the part with the letters "ST GO!" is face up, it was not in the previous one. (00:03:05)
Continuity mistake: In the first shot of the pediatric ward you can see on the left aisle a plush ladybug toy. When Arthur drops the gun, the toy suddenly between cuts is found in the arms of a patient, and then on the bed by him. (00:28:30)
Continuity mistake: When Arthur Fleck gets hit by the yellow sign, the sign breaks in two different ways in the two separate shots - the first time it is evident that the bottom half of the sign splits further in the middle, while in the wider angle it stays largely intact. (00:02:50)
Continuity mistake: When he gets hit by the sign, the round spinner on the back of the sign disappears. (00:02:50)
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.
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)
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)
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)
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)
Plot hole: Given Arthur's mother was placed in an asylum it seems unlikely she would've been allowed to keep him once released. The state would've placed him in another home.
Suggested correction: Nobody says she raised him as a child. As soon as he turned 18 he could have chosen to live with her again. Quite possibly because he didn't like his foster home.
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: Arthur's cuffed in the police car, but he's uncuffed once laid out on the hood.
Continuity mistake: At the end when Arthur/Joker dances on the police car's hood in front of the crowd, in the shot behind him he keeps his arms down, but when it changes into a frontal shot on him he keeps them lifted up.
Continuity mistake: When Joker confesses that he was the one who killed the three guys in the subway at the Murray Franklin Show and also says "It's not a joke", he keeps his gaze turned towards Murray in one shot and towards the audience in the next.
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: 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: 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)
