Continuity mistake: Batman is analyzing Scarecrow's mask at the computer. During the whole scene the photo of his parents and the newspaper change position. (00:10:20)
Continuity mistake: Scarecrow just terrorized the security guard. His two goons approach him asking what did he do. The one with the grey coat has his Tommy gun in his right hand, the one with the brown coat keeps it off frame. In the second they are off camera, grey coat switched hands to the gun, and brown coat is holding his to his chest. (00:04:20)
Continuity mistake: A security guard is recording a silly audition tape-like message. His shadow reaches barely the bottom of the wall, then it's suddenly almost mid-height on it. (00:02:30)
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.
Continuity mistake: Thorne's henchman that stayed behind to check out the aftermath of the explosion is wearing a flower on the lapel of a white coat the first time we see him, and then a gray coat with no flower when he sneaks away to report to his boss. (00:10:15 - 00:11:20)
Continuity mistake: Grace puts down the coffee mug. When "Detective Leopold" shows her in close-up the beeper, the cup has turned around, with the handle where it would have been visible earlier (but it was not). (00:06:50)
Continuity mistake: When Bruce tears away from a clip a picture of Harvey, the clip was part of a clipboard with other papers underneath that, that are now gone. (00:06:00)
Continuity mistake: Thorne bangs angrily his fist on the newspaper with the article about the robbery. The letter opener bounces and stops, but in the next shot it's gone. (00:03:55)
Continuity mistake: When the twins enter the bookie's cashier spot (through an unlocked door), the money is scattered differently on the counter between shots. (00:03:00)
Continuity mistake: Once Big Bad Harv identifies himself and tosses Thorne against his goons, the dossier is gone from the desk as he poses all menacing, and is back there later. (00:18:10)
Continuity mistake: When "Big Bad Harv" is summoned by the shrink and grabs the coin he tossed, the cuff of the shirt is red instead of white. (00:10:50)
Continuity mistake: As evidenced by the news report that Thorne is watching, Carlos and a photographer by him are looking at Harvey's aggression from two different places practically at once. (00:06:10)
Continuity mistake: Carlos' suspenders are not drawn in the shot before he starts clapping - and the ruffian is getting dragged to the police van. (00:04:45)
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.