Deliberate mistake: At Pete's diner, the owner puts the tray down, and instantly glasses appear in front of the 5 patrons. (00:02:50)

Batman: The Animated Series (1992)
1 deliberate mistake - chronological order
Starring: Mark Hamill, Kevin Conroy, Efrem Zimbalist Jr., Loren Lester, Bob Hastings
Genres: Action, Animated, Crime, Drama, Family, Fantasy, Mystery

Continuity mistake: The broken window pane on the door to the District Attorney's office changes slightly from shot to shot. When first shown, the break in the glass extends all the way up to the letter "L" in the word "law." When the shot changes, the break in the glass is nowhere near the letter "L." Also, the letter "W" in the word "law" is missing entirely, even though the break in the glass suggests a portion of the letter would still be intact if the letters were painted on the glass, as is standard. (00:10:25)
Christmas With the Joker - S1-E38
Joker: Jingle bells, Batman smells. Robin laid an egg. The Batmobile lost a wheel and the Joker got awaaaaaaaaayyyyy.





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.