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: 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)
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)
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)
Other mistake: Batman leaps off a tall building carrying Stromwell and lands on one foot without breaking the fall. (00:12:25)
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)
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)
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)
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.