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.
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)
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)
Be A Clown - S1-E11
Continuity mistake: Joker splats a full cream pie on the mayor's face. When he walks away, the screen has mostly cleaned itself. (00:03:40)
Be A Clown - S1-E11
Continuity mistake: The Mayor is giving an inauguration speech. Look at the crowd in the long shot. It cuts closer, and you can see that the man in cowboy hat that was in the crowd moved behind the mayor on stage. A journalist in red tailleur also switches to lilac. (00:01:20)
Continuity mistake: Alfred is skilfully dancing while sitting and sipping tea. The radio rests next to a circular monitor on the computer. Then on a featureless spot, then directly on the monitor itself. (00:04:05)
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: 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 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: 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)
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.