I've Got Batman in My Basement - S1-E20
Other mistake: Toss a brick into a pile of empty boxes, and they won't fly around like it is shown. The heavy brick will sink into whatever hits and possibly other boxes will be affected; here the brick does not deform the box it hits and every single cardboard acts like bouncy balls. (00:13:30)
I've Got Batman in My Basement - S1-E20
Other mistake: The Batmobile moves because Roberta is pushing the gas pedal...but one of the first shots when we see the car move, shows Roberta rolling to the back of the cockpit because of the acceleration. The car does not slow down or anything, though. (00:09:35)
I've Got Batman in My Basement - S1-E20
Other mistake: Penguin and the goons take an impossibly long time to get to Batman once he's incapacitated by the gas and is semi-unconscious at the railing. (00:07:40)
I've Got Batman in My Basement - S1-E20
Other mistake: Sherman touches by mistake the "OFF" button...which turns the conveyor belt on (there is a separate ON button...for some reason). Not just that; the buttons also look completely different between shots; first they are circular buttons with colored lights on top, then they are nondescript small levers. (00:06:55)
I've Got Batman in My Basement - S1-E20
Other mistake: The kids manage to drive away with the Batmobile simply operating the gas pedal, but they show that the gear is manual and it has three pedals.
Other mistake: The Stromwells are hugging in front of the office building, but that was the starting point 5 minutes before, there has been a long chase since. (00:21:15)
Other mistake: Batman leaps off a tall building carrying Stromwell and lands on one foot without breaking the fall. (00:12:25)
Other mistake: After the level crossing, the back of Stromwell's car is missing entirely the fender or any sort of lights. It's a close-up, even. (00:05:45)
Be A Clown - S1-E11
Other mistake: The cards that Jekko the Jokester throws at Batman have in one shot ("Now it's my deal") numbers that do not match at all the symbols. (00:13:40)
Be A Clown - S1-E11
Other mistake: When Jordan comes out of the truck, something went wrong with perspective and/or proportions, because with his leg bent he is practically as tall as the truck door, and he's a child.
Other mistake: Bruce remembers who he is, which somehow gives him also the strength to bend metal (!) and escape. A new prisoner is carried in the camp by the guards, but...he's not new at all. He was already there the first day, standing in the group of prisoners that was eating. And the guy who escorts him is the same that less than a second later spots Bruce from the watchtower and blows the whistle. (00:06:20 - 00:14:30)
Other mistake: The kids need sunglasses to be in daylight, since they have been segregated into the darkness of the sewers for so long. For the happy ending though, only the first one has a moment (very brief, considering that Frog still needed them at Bruce's) of squinting, eye-rubbing and discomfort. The others are instantly fine already the moment they are pulled out of the manhole. (00:21:35)
Other mistake: Batman's lungs have plot armor; he has all the time to put his mask on even after exposure to the gas, and he reacts to it coughing at first. Nobody else in the episode reacts with a cough. (00:07:55)
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)
Other mistake: Batman sees the robbery happen, but he has yet to start his chase when the rotund old lady is deep into discussion with the policeman. (00:03:00)
Other mistake: At the beginning of the episode, the truck of the "Gotham Globe" delivers to the newsstand a nice bunch of copies of the newspaper, which naturally bears the name..."Gotham News" (00:01:20)
Other mistake: Gordon has some impressive ceiling fan in his office, since not only one of its blade supports a full-grown adult hanging by the back of the neck to it, but it even keeps working as usual during it. (00:21:30)
Other mistake: The Scarecrow and his goons escape on an airship, which has the control bridge in the gondola under the main structure. Batman reaches the top of the airship, and so does Anthony the goon...but he does it from a trapdoor on the roof of the gondola. There's a whole structure in between he skipped. (00:15:10)
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.