Goofs

In one of the scenes, Joker is running in his big clown shoes and a second later he's running in his black street shoes.

Arthur shoots the 3 men on the subway, he is using a Smith and Wesson Chief Special 38 that holds 5 rounds. You can count a total of 8 shots during the scene, but he does not reload.

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.

The Joker is in the back seat of police car #9189, which is damaged significantly in a crash a few minutes later. One of the 2 police cars shown responding to the crash is #9189.

As Arthur is standing on the cop car after the crash with the ambulance, someone in the mob is holding a bat right in front of Arthur. The shot changes and there are no bats in the air in front of him anywhere.

Arthur fires eight shots with a 5-shot .38 revolver without reloading when he kills the Wall Street Three.

Modern refrigerators, including the one that Arthur Joaquin Phoenix empties and then steps into, closing the door behind him (at approximately 1:07:50), use magnetic door seals. Their doors are opened merely by pulling on them. Arthur would have no trouble pushing the door open from inside the refrigerator.

The idea that a late night talk show would air a video of a comic from a club just to make fun of his act in 1981 is absurd. This type of thing did not take place until the modern smart phone age. In 1981, they would have had to have permission from both the comic and the club, and the act of airing it without permission likely would spark a lawsuit.

This is not necessarily a plot hole. Since the act was taped in the club it could be reasoned that Arthur had to sign the rights for the publication to the club beforehand. The club could then give permission to the station broadcasting the late night show. This may seem far fetched for the real world, but keep in mind that the movie still takes place in a fictional world, which may have some different laws.

When the rioters pull Arthur out of the police car, he is not handcuffed.

The sign with "everything must go" breaks a second before it hits Arthur.

When Joker awakens on the hood of the police car, he's clearly laying on a fake background, as the size, shape and dents are very different than the actual hood shown in the next wider shot.

In the final shot, as he's leaving the doctor's office, trailing of bloody footprints, the third step of his left foot doesn't leave a mark.

When Arthur's neighbor, Sophie, enters the elevator with her daughter, you can see her press an elevator button. If she and Arthur live on the same floor, he is expected to have already pressed the button for the floor they live on.

When Arthur goes to the public theatre to confront Thomas, there is a shot of the stage with two speakers visible on either side of the stage. These speakers are modern rigged speaker arrays, which were not yet used in the 1970s.

This film takes place in 1981. The Energizer Bunny is shown in TV advertisements, but in real life, the mascot was not introduced until 1988.

Set in 1981, there is an Kellogg's ad playing with Keanu Reeves on a TV/digital billboard. This ad did not come out until 1987.

A sign in a shop window declares "Cell phones fixed here". Cell phones did not exist in 1981. The earliest cell phones were introduced in 1983 and it would be several more years before such signs became commonplace.

As the police car pulls out of the studio and makes a turn, an empty NYC Citi bikeshare rack can be seen on the right side.

When Arthur is taking a smoke break outside the hospital and the detectives confront him, he gets up and tries to re-enter the hospital, but the automatic door doesn't open. Arthur waves frantically at the motion sensor atop the door trying to get it to open. However, motion-sensing automatic doors were extremely uncommon in 1981; the older bottom mat weight sensor-based "supermarket door" style would have been prevalent for automatic doors.

After Arthur steals his mother's file from the clerk at the mental hospital, he races down the hallway and turns toward the stairs. You can see the stuntman's face for a split second; it's clearly not Joaquin Phoenix.

Security at television studios, especially with live audiences are renowned for being very strict, even in 1981. Arthur's gun would have been flagged and confiscated when he arrived for his appearance on the Murray Franklin Show.

As seen in her file, there are newspaper clippings about Penny and her boyfriend tying her adopted son Arthur to a radiator and other abuses. With events like this serious enough to make the newspapers, it is very doubtful that authorities would allow her to keep the child.

It's never shown how the hospital or the police respond to the sudden death of Penny, after Arthur smothered her. Gary and Randall say that they've heard about the death, but don't say how they heard.

In light of what a target he has become, it defies credulity that Thomas Wayne and his family would attend the theater amid all the chaos, and he would not have security protecting them.

The theater Wayne takes Bruce to is showing "Blow Out" and "Zorro: The Gay Blade", two movies which were intentionally not made for children.