Continuity mistake: The henchmen spot Stromwell as he makes a run for it through the train yard. Before the chase begins, Thorne gets in frame in a close-up, and he has eyebrows matching the white of his hair instead of his usual black. (00:17:15)
Continuity mistake: When we first meet Harley Quinn, her gloves are white before becoming their normal colors of black and red in the next shot.
Continuity mistake: The Joker reads “Charles Michael Collins” from Charlie’s card, but later, when he shows it to the camera, there is no middle name on it.
Continuity mistake: A newspaper clipping shows Bruce and Harvey shaking hands in front of a sign. The sign in the photo is different from the sign they were actually standing in front of. It says simply "Future site of Gotham Penitentiary" instead of "of the new Gotham Penitentiary." (00:02:10)
Continuity mistake: The waiter that brings the snails to Pamela brings them in a tall bowl-like container in the wide view, but it's a normal plate in the next shot. (00:04:10)
Continuity mistake: The dessert on the table changes completely in appearance from the start of the scene (Pamela bids her farewell) to the rest (Harvey does not feel good). (00:06:40)
Continuity mistake: When Pamela kisses Harvey at the cafe, her left hand is on the shoulder first, then the top of the head in the next shot. (00:06:50)
Continuity mistake: Harvey gulps down the whole glass of water, but when Bruce tries to revive him after the chocolate mousse incident both glasses of water on the table are still full. (00:07:50)
Continuity mistake: Harvey Dent is brought inside the hospital on the stretcher fully dressed in his business suit. Which is pristine clean, despite the face he just facemoussed and got chocolate splatters all over himself. (00:08:40)
Continuity mistake: Bruce Wayne is holding the microscope slide in two different ways in the two different shots. (00:10:30)
Continuity mistake: When she says "Mm, a little big for a fly" at the sight of Batman, Poison Ivy is wearing her boots and bodice, but her legs are bare. In the rest of the episode she is wearing green stockings. (00:16:15)
Continuity mistake: Batman breaks a few teeth of the monstrous venus flytrap-like plant, but it has a mouth still full of teeth in the rest of the episode. Similar fate befalls Batman's cape. (00:16:20)
Continuity mistake: When Batman cuts off the first tentacle, the liquid is clear. Further slashes reveal the 'blood' of the creature as being green-ish. (00:18:40)
Continuity mistake: When Poison Ivy advanced towards Batman that is clinging to the edge of the trapdoor, she is empty-handed at first, then all of a sudden she holds the antidote. (00:20:15)
Continuity mistake: By the time Batman gets to Ivy's garden, his cape and hood are blue, but turn to black a few seconds later.
Continuity mistake: When Poison Ivy is introduced in her trademark green costume, she's bare legged. Yet a few shots later, she's suddenly wearing green tights. While she's offscreen for a brief period, there's not enough time between the shots for her to have put them on.
Continuity mistake: Doctor Long pushes a button on the elevator's board. The key shines bright in close-up, it's not lit in the wider angle. (00:02:30)
Continuity mistake: Scarecrow just terrorized the security guard. His two goons approach him asking what did he do. The one with the grey coat has his Tommy gun in his right hand, the one with the brown coat keeps it off frame. In the second they are off camera, grey coat switched hands to the gun, and brown coat is holding his to his chest. (00:04:20)
Continuity mistake: Both Batman and Scarecrow's henchmen are reading the same newspaper, one with a "University Museum Benefit Tonight" headline. The same headline though has a different layout; the words "museum" and "benefit" have bigger spacing in Bruce's copy. (00:10:15)
Continuity mistake: Batman is analyzing Scarecrow's mask at the computer. During the whole scene the photo of his parents and the newspaper change position. (00:10:20)
Answer: Because he stole a chemical from his lab.
But Batman knew that with it, Clayface could permanently retain his shape and Clayface desperately wanted to be Matt Hagen again. In a sense, Batman stopped Clayface from becoming human.