Heart of Ice - S1-E3
Other mistake: Batman uses a thermos full of chicken noodle soup to defeat Mr. Freeze, and moments later gives Summer Gleason a security tape that incriminates Ferris Boyle. When Batman pulls out both of these aforementioned items, there is a corresponding clicking sound that suggests he pulls them from his utility belt, but both items are far too large to fit inside the belt. They also could not have been simply been hanging from his belt, as Mr. Freeze had possession of Batman's belt a few minutes prior and they were not there. (00:20:05 - 00:20:45)
Continuity mistake: When Clay-Face makes things like crab claws they're red. Yet later on when he fights Batman again in the TV station they're light brown.
Continuity mistake: The clay used to bind Germs goes from a creamy light brown to dark brown.
Continuity mistake: Clayface has clay in the shape of the right hand on his chest when fighting Batman. A few seconds later it's suddenly in the shape of the left hand.
Character mistake: The news program Stromwell is watching at the beginning of the episode displays a picture of Commissioner Gordon with the caption "Comisioner." (00:02:00)
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)
Deliberate mistake: At Pete's diner, the owner puts the tray down, and instantly glasses appear in front of the 5 patrons. (00:02:50)
Revealing mistake: The cane of the 'mysterious' drunk that happens to be not too far from Thorne's table is hovering instead of being properly hanging off the edge of the table in a couple of shots. (00:03:00)
Audio problem: At the end of the first flashback, Stromwell's car is shaking long before the engine revs. (00:05:40)
Other mistake: After the level crossing, the back of Stromwell's car is missing entirely the fender or any sort of lights. It's a close-up, even. (00:05:45)
Continuity mistake: Thorne's henchman that stayed behind to check out the aftermath of the explosion is wearing a flower on the lapel of a white coat the first time we see him, and then a gray coat with no flower when he sneaks away to report to his boss. (00:10:15 - 00:11:20)
Other mistake: Batman leaps off a tall building carrying Stromwell and lands on one foot without breaking the fall. (00:12:25)
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: 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: 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)
Other mistake: The Stromwells are hugging in front of the office building, but that was the starting point 5 minutes before, there has been a long chase since. (00:21:15)
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.
Factual error: The note the Joker has that says Charlie has changed his name to Don Wallace lists his address as 12 Marigold Lane, Springdale, Ohio, a city near Cincinnati; however his area code is listed as 614, which is the Columbus, Ohio area code.
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)
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.