Continuity mistake: Arkham Asylum cells are different in different episodes - in Fear of Victory and Fire From Olympus they come with big windows. Whereas in others like Dreams in Darkness and Harlequinade they're smaller with small windows in the doors that are sliding open kind.
Continuity mistake: In some episodes like Trial the inmates are wearing prison uniforms while in others like Fear of Victory they're wearing their costumes. Another thing is the prison uniforms constantly change color, in some episodes they're light grey, others light blue, then in some light purple.
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.
Other mistake: On the police blimp, the scanner/radar screen in close-up has a single white circle, but in the wider angle it's two black circles, which are even hand-drawn and thus not perfect circles. (00:01:30)
Continuity mistake: A security guard is recording a silly audition tape-like message. His shadow reaches barely the bottom of the wall, then it's suddenly almost mid-height on it. (00:02:30)
Other mistake: Batman is reading the front page of the newspaper, but in the drawings that follow, the paper is right-most, it opens to the left of the page. Gotham is not in Japan. (00:04:50)
Character mistake: When Batman looks up at the computer the data of the burglaries, he says they happened "this week", but the dates are June 28, July 3, and July 13. On the screen there's also a typo; "Buaglary." (00:05:00)
Continuity mistake: Batman goes in detective mode in the room, and scans it for clues. The shattered object on the desk changed size and placement of the pieces. (00:07:50)
Other mistake: When Batman listens to the tape left behind by the security guard (and the police's incredibly sloppy investigation) he presses play and not only the tape is at the right point without him having to rewind, but the recording is much different from the actual events and gives him a convenient abridged version. (00:08:00)
Continuity mistake: In the bat nerds laboratory, Bruce Wayne shows them the tape; his hand suddenly turns between shots. (00:12:55)
Continuity mistake: When Batman smashes his face against the blimp, in the shot from inside the cockpit there's not much damage, while in the reverse shot it shows how Batman's pointy chin created a rather large web-pattern of damage. (00:18:30)
Character mistake: When Batman uses his magic computer to analyze Langstrom/Man-Bat, lots of lovely gibberish appears on screen, with some notable typos such as "Bat membaaanes", "Sedum Chloride D", "Mammac higi Index Gat harmal nomeaace at padiotion." Your guess is as good as mine for the last one. (00:21:10)
Heart of Ice - S1-E3
Other mistake: When the children interrupt Summer Gleason's news report to play in the snow, the two police officers on the right are completely motionless for the entire 15-second shot. (00:02:10)
Heart of Ice - S1-E3
Continuity mistake: When Mr. Freeze's henchman Johnny gets his legs frozen, he is wearing pants and shoes. Later when Johnny is inside a water tank in the Batcave in order to thaw the ice, he has been stripped down to his boxer shorts. While it's possible for Batman to remove Johnny's pants above the freeze-line by cutting them away, he would still have remnants of his pants underneath the ice, but when the ice is thawed, his legs and feet are bare. (00:05:45 - 00:08:05)
Heart of Ice - S1-E3
Continuity mistake: When Mr. Freeze's henchman Johnny gets his legs frozen, an ice "bridge" connects the gap between his feet. A few shots later and the connective ice is gone, making it look like he is wearing boots made of ice. (00:05:45 - 00:07:00)
Heart of Ice - S1-E3
Revealing mistake: When Batman - disguised as a security guard - opens his utility belt, the belt opens and shows a configuration of buttons and a split-second later, a key card materializes out of thin air in front of the buttons. (00:11:30)
Heart of Ice - S1-E3
Other mistake: When Batman is watching the security footage of Victor Fries' accident, at first Fries is addressing the camera directly which follows him and pans according to his movements and Ferris Boyle interrupting him, which would suggest a second person was in the room with a handheld camera. But after Ferris Boyle enters the room and orders Fries' experiment terminated, there are several cuts from various angles in the room suggesting surveillance cameras. The surveillance style footage also often conveniently cuts to the face of whichever person happens to be speaking or will cut to a certain angle in order to put emphasis on the action, all of which would require multiple mounted cameras and purposeful editing by either Boyle or one of his subordinates, which would be completely illogical since they don't want anyone to see the footage in the first place. (00:12:20 - 00:13:55)
Heart of Ice - S1-E3
Continuity mistake: In the wide shot of Batman hanging upside down in Mr. Freeze's lair, his face is blank and the bat outline from his chest emblem is missing. (00:14:10)
Heart of Ice - S1-E3
Continuity mistake: When Batman breaks free of his ice handcuffs, the colors on his chest emblem are inverted. (00:15:30)
Heart of Ice - S1-E3
Continuity mistake: When Mr. Freeze is running towards the fire hydrant, it is partially covered in a layer of ice. When the shot changes, the ice has disappeared altogether. (00:18:15)
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.