Robin's Reckoning (2) - S1-E53
Continuity mistake: When Batman ties a wooden beam to his injured leg, it extends up past his knee, but later during his fight on the merry-go-round, it stops below his knee.
Robin's Reckoning (1) - S1-E51
Continuity mistake: During the flashback, Batman's suit has a full black bat emblem on it without the yellow circle, but in the scene where he flips one of the gamblers over his shoulder, the black bat changes to the one with the yellow circle.
Continuity mistake: When Bell and Germs carry Hagen to his car, after force feeding him the Renuyu formula, Hagen is wearing a full brown suit, but after Bell and Germs place Hagen inside the car, his jacket is now blue.
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.