I've Got Batman in My Basement - S1-E20
Continuity mistake: Batman looks at the vulture after outwrestling it, and then, sprints to get his 'rang back. When he looks out he's got a cut on the suit, on the left, but when he fetches the weapon the torn part is gone. Cut away, and now the tear is back and doubled over to the other side. (00:02:55)
I've Got Batman in My Basement - S1-E20
Continuity mistake: Sherman hangs the two front pages overlapping in close-up, at distance in the next shot. (00:21:15)
I've Got Batman in My Basement - S1-E20
Continuity mistake: When Mrs. Grant gets in the basement at the end, the Penguin is crushed under a long plank, different from the shelf that dropped on him earlier. (00:20:40)
I've Got Batman in My Basement - S1-E20
Continuity mistake: When the Penguin melts the front door lock, the holes on the door change distance between shots. (00:16:35)
I've Got Batman in My Basement - S1-E20
Continuity mistake: Mrs. Grant casts a shadow on the stairs that reaches a different point in different shots even if she did not move. (00:12:35)
I've Got Batman in My Basement - S1-E20
Continuity mistake: Batman and the kids approach the exit; you can see Roberta touching the push handle, but in the next, closer, shot they have yet to get there and Batman kicks the door open (without trying the panic bar). (00:07:25)
I've Got Batman in My Basement - S1-E20
Continuity mistake: From below, Sherman's red hair is visible, so he is on the left side of the sack of seed. In close-up though, he's in the middle, by the hole it leaks seed from. (00:05:15)
I've Got Batman in My Basement - S1-E20
Continuity mistake: The talon cuts on Batman's cape change in space and position between shots; they are firmly on top of the shoulders at the end of the struggle, but they were lower during the actual fight. (00:02:20)
I've Got Batman in My Basement - S1-E20
Continuity mistake: The size of the egg the Penguin's guys steal changes throughout the opening. It's twice as big as the ginger's hands when he holds it, but passed through a small triangular (for some reason) section they cut in the window, and when Batman reveals himself the egg is smaller.
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)
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.