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)
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)
Continuity mistake: Thomas Wayne and Arthur Fleck are talking in the bathroom. Joaquin Phoenix's hair changes position on his forehead several times even when he does not touch it and/or shakes his head. (01:06:15)
Continuity mistake: During the scene when Arthur is making funny faces at the kid, the hair wisps on Joaquin Phoenix's forehead change position between shots. The mother also turns slightly towards him in almost every cut, but his antics are in continuity. She keeps 'just noticing' him and turning towards him without the opposite movement to balance. (00:07:45)
Continuity mistake: In the close-up of the gun at the beginning of the scene when Arthur is watching "Slap that bass" from "Shall we dance", notice in background the bullets; there is one angled parallel to the gun and 3 more at a slightly different inclination, plus one at right angle with the first (plus other 2 not quite aligned with the rest). A couple seconds later he aims the gun at Fred Astaire, and that group of bullets changed angle entirely. (00:22:10)
Continuity mistake: Arthur gets on stage at the improv club and has one of his laughing fits. When the camera is behind him, facing the blinding stage lights, you can see that he is choking himself with the right arm and holding the book with the left. But it was the exact opposite in the previous shot. (00:43:44)
Continuity mistake: The last of the Wall Street Three is shot as he tries to flee; he falls down in two very different takes. In the side view he choreographically falls with the jacket rolling up his back, and with the left leg (initially) forward, foot on the ground. In the previous view, camera behind Joaquin Phoenix, he fell with the sole of that foot kicked back, and with the jacket still down his back. (00:32:25)
Continuity mistake: Arthur finishes the last yuppie off, and he lies stone cold dead on the stairs. However, he is not lying in the same position the two times it is seen. The second time he has his left hand one step lower than the right (which was the only one visible earlier) and his knees are on the platform (in the first shot, only the tip of the shoes touch the floor). (00:33:45)
Continuity mistake: When one of the Wall Street Three charges the punch to hit the not-yet-Joker, his buddy in the background holds Arthur's bag with the hand on top, to the point that right before the cut his thumb sinks in the bag making it bend. When the punch lands, he's holding the bag by the side. (00:32:15)
Continuity mistake: Arthur tries to get up from his seat, but one of the yuppie bullies restrains him. Then the lights flicker for a moment, but it's an editing trick, as something else changed in the car; the chips' paper bag that was sitting on one of the seats in the row facing Arthur's, is suddenly gone. It comes back right after, when Arthur is struck down by the punch, and it keeps coming and going. For instance; Joker shoots the second guy dead? Bag's on the seat. Third guy runs away? Bag gone. (00:32:05)
Continuity mistake: One of the Wall Street Three begins approaching Arthur, and doing so he grabs the vertical support and spins around it. There's a cut mid-spin and his buddy in the background is standing in different positions; first he is laughing leaning against the support closer to him, then he is in the middle of the subway car holding two poles. (00:31:20)
Continuity mistake: Talking with his social worker Debra Kane, Arthur is moving his legs nervously as she asks him about the journal. In that shot his cigarette is almost smoked to the filter, but in the next one there's still an inch to go. (00:05:35)
Continuity mistake: Thomas Wayne is speaking on TV. Arthur shushes his mom. In the close-up that follows, suddenly there's an extra lock of hair against his forehead. (00:38:50)
Continuity mistake: When Arthur is choking Alfred Pennyworth through the bars, in the shot when he says "Let go, let go!" you can see behind his head the flowers from the magic wand, absent in the previous views. (00:54:50)
Continuity mistake: During the opening credits, while the main character is clowning around with the sign the same passersby (in particular two women - one with a blue coat, the other with a plain light brown one) keep walking past him in multiple shots. (00:01:35)
Continuity mistake: The camera pans towards Arthur when is reading Penny's letter to Thomas Wayne; the items on the table (newspaper, pen) are in a different position compared to the rest of the scene. (00:48:35)
Continuity mistake: In a close-up, Arthur grabs with both hands the gun he dropped and kicked across the hospital floor, but in the wider shot that follows he's using only the left hand, and it's also clear that Joaquin Phoenix's body is turned in a complete different way, with his feet adjoined instead of the open legs seen before. (00:28:35)
Continuity mistake: In the theater lavatory, Arthur puts the usher jacket in the sink. Part of the jacket drapes to the side of the sink, but at the cut the cloth is entirely contained in it. (01:05:00)
Continuity mistake: Arthur inadvertently fires a gunshot in his living room. Right as he tumbles across the table, he knocks off the table the paper bag the gun came with, and knocks towards the edge of the table a pill bottle. In the closer angle shot when he cranks up the volume of the TV set, the bag is still on the table and the bottle is gone. The newspapers are also positioned differently. In the next shot we're back to the previous state (pills up, bag down, newspapers closer together). (00:23:10)
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)
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.