Christmas With the Joker - S1-E38
Other mistake: Batman and Robin are walking from the elevator to the Batmobile. Due to a coloring error, Robin's whole chest is green instead of red. (00:02:15)
Christmas With the Joker - S1-E38
Revealing mistake: Batman and Robin, thanks to the Joker's generous hint, are at the toy factory. In the wide shot just before the phrase "Look sharp, Robin", look at the shadow on the ground; Batman's cape deflates, but the shadow does not move and is still just as wide as it was before. (00:17:10)
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)
Christmas With the Joker - S1-E38
Other mistake: The train falls off the cliff, explodes, but during the explosion we see bounce off the flames...the train, perfectly intact, even with the lights on. (00:11:20)
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: 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
Other mistake: Joker hosts the show in front of an audience of plywood mock-up figurines. In the wider views we see next to Gordon... Alfred. That would require Joker to know Batman's secret identity. (00:06:45)
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: 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
Other mistake: Once Joker's back in his Arkham cell he should be wearing prison pants again like he was at the beginning of the episode. Yet he's still in his purple pants.
Christmas With the Joker - S1-E38
Other mistake: Robin is trying to change channel but is unsuccessful, because the Joker hijacked all transmissions. Perhaps also because the remote control he has in hand has just one button. (00:04:40)
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.