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)
Character mistake: The French spoken by the waiter of the cafe is phony in a pretty atrocious way for a supposed high class restaurant. Can't even pronounce "monsieur" right, it's merely a tacked-on accent. (00:04:10)
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)
Revealing mistake: Harvey Dent has been poisoned. Commissioner Gordon picks up the phone; there's a mistake in the animation here, with the telephone cord clipping through. (00:08:50)
Continuity mistake: Bruce Wayne is holding the microscope slide in two different ways in the two different shots. (00:10:30)
Character mistake: Poison Ivy tells her beloved plant "I won't clip off any more of your beautiful petals." She clips off the whole flower, not really "petals." (00:14:35)
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)
Other mistake: Melissa Manchester and Neil Ross voice minor characters in this episode. The end credits list them like this though; "Additional Voices - Melissa Manchester as Neil Ross." It should have been "And", of course. (00:21:55)
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.
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.