When Harley leaves the cell area after freeing Cassandra, she's soaking wet from the sprinklers. When she next appears, in the evidence room looking for Cassandra, she's dry.
In the grocery store, the contents of Harley's cart change between shots.
When Harley Quinn is hiding behind the big pile of drugs, it gets shot up and the powder floats through the air. In later shots, though, there is no powder visible on the ground near the drug pallet.
Towards the end of the movie when the guy in the passenger seat is shooting at Harley, who is skating in front of the car, he is hanging out of the window. When she flips to the top of the car, the door is suddenly open.
Detective Renee Montoya holds the evidence bag with the J necklace in a different way between shots.
When Black Canary sings in the club, her voice breaks a martini glass which is partially full. While finding the perfect resonance to shatter an empty glass may be possible, adding liquid changes the critical resonance of the submerged portion. This acts to dampen vibrations, making it impossible to shatter from a single voice.
In the film Harley Quinn says that she holds a PhD and is a psychiatrist. In order to become a psychiatrist you do not get a PhD. You would get a PhD if you wanted to become a psychologist. Psychiatrists are physicians who are able to prescribe and treat mental health disorders with medicine versus a psychologist who treats a patient with therapy. Anyone wishing to become a psychiatrist in the United States would first need to attend either medical school or osteopathic medical school after earning an undergraduate degree. After four years the potential psychiatrist earns either a Doctor of Medicine (MD) or Doctor of Osteopathic Medicine (DO) degree. After graduating and spending an additional four year in a psychiatric residency the resident can elect to pursue a sub specialty fellowship for an additional year (in fields such as addiction, geriatric, forensic, consult liaison) or two years (child & adolescent psychiatry), After obtaining a state license the psychiatrist may now begin practicing. So Harley was not a physician.
After buying an egg sandwich, it focuses on the fact she is wearing one flat shoe and one heeled boot. She is then chased by the police and is wearing two flat shoes from then on. This is a running gag as later in the film, Black Canary comments "When the fuck did she have time to do a shoe change."
At two different points during the movie, Harley turns on her TV and the same clip of a Tweety cartoon is playing.
Several times during fight scenes, especially at the police station, many of Harley's opponents are seen just standing idly by doing nothing while she's finishing off whoever she's fighting, before it's their turn to have their a$$ kicked.
Incorrectly regarded as a mistake: "Burying the lede" is actually the correct saying. The words "lead" and "lede" are homonyms, meaning they are spelled differently but pronounced the same.
At 0:37:39 -Sionis says to Black Canary: "Talk about burying the lead" but the closed captioning reads: "Talk about burying the lede".
From 0:39:12 -certainly the fastest "arrest" in history.
Several times the Huntress's projectiles are called "arrows". Although a crossbow's projectiles are arrow-like, they are known as bolts or quarrels.