Continuity mistake: When Captain Boomerang kills his accomplice, he falls to the floor, and his money bag is in his left hand. Then it is shown on his right side.
Continuity mistake: While the Squad is patrolling the city, the ends of Harley's bunches keep changing in style; they are straight when she's changing at the prison and on the helicopter with the Squad, really curled after Rick kills someone, then jump between straight and slightly curled for the rest of the film, with the exception of the end scene where she hasn't got her hair in bunches anymore, but a different hairstyle.
Continuity mistake: When Harley is in the elevator when the doors open you can see that her blue hair piece is coming off, but when they cut to the next shot it's secure and tight to her head.
Continuity mistake: In the extended cut of the movie, during the final fight with the Enchantress, Harley Quinn hits her in the head with her baseball bat. A couple of shots after that are inverted, you can see it because she should be wearing the blue part of her pants and ponytail to her left side, but in these shots, the blue side is the right one.
Continuity mistake: After Harley hits her head against the bars of the cell, her body and face change position between shots.
Answer: Because he's already been into the liquid, it's how he became the Joker. He wanted Harley to be just like him.