Christmas With the Joker - S1-E38
Continuity mistake: The ribbon on top of the big package left by Joker by the antenna is inconsistent in position between shots. (00:12:00)
The Man Who Killed Batman - S1-E49
Continuity mistake: Rupert Thorne's eyebrows are briefly white in one shot when he's talking to Sidney.
Continuity mistake: During his introduction Scarface's scleras go from light yellow to white then back to light yellow.
Continuity mistake: When Scarface's gang goes to steal the platinum from the ship and burn a hole in its hull, there is already a stack of platinum bars in their boat before they enter the ship.
Continuity mistake: Batgirl's cowl, cape, gloves, and boots are black in this episode, but in Shadow of the Bat Part II and Batgirl Returns, they are now blue and remained that way onwards.
Continuity mistake: Two-Face's left leg is blue here whereas it was normal in part 1 of his origin episode.
Continuity mistake: When Riddler burns his costume, he's wearing his purple gloves, but after throwing his green jacket into the fireplace, his gloves disappear entirely. (04:25:03 - 04:29:59)
Continuity mistake: Riddler's gloves disappear briefly after Batman confronts him at his party.
Continuity mistake: In the overhead of Dr. Leland's office at the beginning, Dr. Leland is missing her face. (00:00:57)
Continuity mistake: Harley's shoes are poorly drawn when she dances in Dr. Leland's office.
Continuity mistake: Both of Harley's shoes are red as Batman holds her after rescuing her from falling.
Continuity mistake: Batgirl and Catwoman briefly gain pupils in one shot of them during the Stacked Deck fight, despite not having any throughout the episode. (04:00:39 - 04:41:24)
Continuity mistake: Batgirl and Catwoman are depicted without pupils, but in one shot during the fight at the Stacked Deck, they briefly gain pupils when Batgirl says "I think it's closing time."
Continuity mistake: Batgirl's cape changes length throughout the episode. It goes from reaching her ankles to her hips.
Continuity mistake: When she was pulling Daggett up, Batgirl briefly gains pupils when she says "Catwoman, wait."
Continuity mistake: The soles of Batgirl's boots change from light to dark blue throughout the episode.
Continuity mistake: Batgirl's boots gain and lose heels throughout the climax of the episode.
Continuity mistake: When Selina dons her Catwoman costume, she pulls on her mask which was detached from her costume like a face mask, but later on when she returns to her apartment after the museum robbery, she removes her mask which is now attached to her costume like a hood.
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.