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)
Continuity mistake: Right at the beginning of the dance in the children ward, the kid with a blue hat on Arthur's right is sitting cross-legged with the hospital robe covering his calves and feet. At the first cut, his jammies and socks become visible. (00:28:05)
Continuity mistake: Murray's blue jacket, when he comes out from behind the curtain his jacket is open, than he makes a twirl and the first button of the jacket is buttoned up.
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)
Continuity mistake: When Arthur imagines that he is on Franklin's TV show the button on Murray's jacket is off then on. (00:12:20)
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)
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)
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'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)
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)
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.