Be A Clown - S1-E11
Revealing mistake: When Mayor Hamilton Hill speaks to his geriatric butler as he is tying up the birthday banner, the butler's right hand is miscolored in some frames and becomes part of the banner, same purple color. (00:03:55)
Be A Clown - S1-E11
Revealing mistake: When the tied up Batman is forced to emulate Houdini, he draws the air that will last him for the two minutes of his act while water already submerged his face. (00:15:15)
Be A Clown - S1-E11
Revealing mistake: At the beginning of the shot with the creepy clown that sends his unicycle crashing and begins juggling, the kids in the background are frozen in place for the first frames, then start moving. (00:05:15)
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.