Christmas With the Joker - S1-E38
Continuity mistake: During the wacky scene of the Joker's escape, the balls on the Christmas tree vanish /change position and size between shots. (00:01:35)
Christmas With the Joker - S1-E38
Continuity mistake: In the scene with the old lady and the lad who brings her the package she dropped, the red stripe on the golden package changed position between shots. (00:03:50)
Christmas With the Joker - S1-E38
Continuity mistake: Joker puts in the mouth of "the Lawful family" members candy canes that they can't spit away. He puts back in Summer Gleeson's mouth the cane with the hook pointing up, but it points down when he's done with Bullock. (00:07:40)
Christmas With the Joker - S1-E38
Continuity mistake: The Joker's goons blew up the bridge, and Joker turns to the three hostages. Bullock's candy cane is facing Summer's, but the Joker put it in his mouth facing the other direction. (00:07:50 - 00:09:40)
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)
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.