Continuity mistake: Scarecrow's shoes are light brown in most of the episode. Yet once he's back in his cell they're suddenly dark grey.
Continuity mistake: When The Joker's going down the garbage chute he's riding down on his butt yet somehow he's on his stomach after a shot of Batman is shown.
Other mistake: Croc's feet are briefly a shade of pink after his sewer fight with Bane.
Continuity mistake: The Clock King slashes Batman's chest logo with his clock hand cane leaving it sliced up. A few shots later it's intact.
Continuity mistake: When Batman stops the countdown it reads as 1:45. When Scarecrow starts it back up it's suddenly 0:14.
The Cat and the Claw (1) - S1-E1
Continuity mistake: In the beginning when Catwoman lands, she's seen putting her whip away. In the next shot looking through the window, she's rolling up her whip.
Continuity mistake: Batman tears off the front of The Scarecrow's collar. Yet later in his hideout it's fine again.
Continuity mistake: Two-Face's left leg is blue here whereas it was normal in part 1 of his origin episode.
I've Got Batman in My Basement - S1-E20
Revealing mistake: Sherman puts down the Jr. Detective box to try to get back the stuff the bullies grabbed. He sets the box on one of the steps, but the way it is drawn defies gravity. (00:03:40)
I've Got Batman in My Basement - S1-E20
Continuity mistake: Batman looks at the vulture after outwrestling it, and then, sprints to get his 'rang back. When he looks out he's got a cut on the suit, on the left, but when he fetches the weapon the torn part is gone. Cut away, and now the tear is back and doubled over to the other side. (00:02:55)
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: Batman throws a chair and shatters a window. You can see that there is basically no part of the frame nor glass left at the bottom, but when Stromwell looks outside and when Batman gets back in the office, part of it is still standing. (00:15:55)
Continuity mistake: Stromwell tosses on the desk the binder with the information; it does not have a label on the cover, then it does. (00:14:40)
Audio problem: At the end of the first flashback, Stromwell's car is shaking long before the engine revs. (00:05:40)
Continuity mistake: Stromwell crushes under his fist the remote control - not bad for a supposed old man! But the remote is also completely gone in the next shot. (00:02:10)
Continuity mistake: There's a marked cone of light on the door that leads to Harvey's room inside club Wild Deuce; it disappears after the opening shot. (00:15:05)
Continuity mistake: When Batman drives his two-wheeled vehicle out of the Batcave, there are no numbers on the concrete as they appeared in 1x8.
Continuity mistake: Harvey Dent's mud stain is different when he looks in disbelief after Thorne's goon says he'll chew and spit him. He's got a stain on his lapel that was never there before and is gone later. (00:04:55)
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.