Continuity mistake: Batman blasts the last door between himself and the villain. The floor is in frame for a lengthy time and there's no trace of debris, but in the next shot Batman passes the remains of the door. (00:20:35)
Continuity mistake: Gathering evidence inside the Sewer King's lair, Batman's camera changes size (it gets bigger) and shape (the button is on the left but it started on the right). (00:15:55)
Continuity mistake: The content of the table of the Sewer King constantly change when he has his messy dinner. At the end he flips the table and throws everything on the floor, and the floor is clean a second after, even. (00:12:35)
Continuity mistake: The policeman that writes down the woman's leprechaun sighting is using a brown and yellow notepad in the first shot, which turns into a white paper sheet in the next. (00:03:05)
Continuity mistake: When the Matron fails to draw the taxi's attention, a couple passes by her (she wears a blue raincoat and a purple beret) but they were nowhere in sight in the wide angle. (00:02:45)
Audio problem: Alfred brings breakfast. He says "Rise and shine", but no mouth animation is included. (00:07:10)
I've Got Batman in My Basement - S1-E20
Continuity mistake: The balls of the bolas that tie Penguin up change position between shots. (00:18:00)
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)
Continuity mistake: When Poison Ivy advanced towards Batman that is clinging to the edge of the trapdoor, she is empty-handed at first, then all of a sudden she holds the antidote. (00:20:15)
Continuity mistake: Bruce Wayne is holding the microscope slide in two different ways in the two different shots. (00:10:30)
Revealing mistake: Harvey Dent has been poisoned. Commissioner Gordon picks up the phone; there's a mistake in the animation here, with the telephone cord clipping through. (00:08:50)
Continuity mistake: Harvey gulps down the whole glass of water, but when Bruce tries to revive him after the chocolate mousse incident both glasses of water on the table are still full. (00:07:50)
Continuity mistake: The dessert on the table changes completely in appearance from the start of the scene (Pamela bids her farewell) to the rest (Harvey does not feel good). (00:06:40)
Continuity mistake: When Pamela kisses Harvey at the cafe, her left hand is on the shoulder first, then the top of the head in the next shot. (00:06:50)
Continuity mistake: The waiter that brings the snails to Pamela brings them in a tall bowl-like container in the wide view, but it's a normal plate in the next shot. (00:04:10)
Continuity mistake: A newspaper clipping shows Bruce and Harvey shaking hands in front of a sign. The sign in the photo is different from the sign they were actually standing in front of. It says simply "Future site of Gotham Penitentiary" instead of "of the new Gotham Penitentiary." (00:02:10)
Continuity mistake: Captain Clown chases Batman with its arms outstretched, except in the long shot with the crane; he is walking with his arms down his sides. (00:15:45)
Continuity mistake: When Captain Clown lifts the drum to toss it into the river, the lid shows no holes; Joker punches a few in it earlier. (00:11:55)
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.