Continuity mistake: Batgirl's cowl, cape, gloves, and boots are black in this episode, but in Shadow of the Bat Part II and Batgirl Returns, they are now blue and remained that way onwards.
Batman: The Animated Series (1992)
1 mistake in Shadow of the Bat
Starring: Mark Hamill, Kevin Conroy, Efrem Zimbalist Jr., Loren Lester, Bob Hastings
Genres: Action, Animated, Crime, Drama, Family, Fantasy, Mystery
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.