Heart of Ice - S1-E3
Continuity mistake: When Mr. Freeze is running towards the fire hydrant, it is partially covered in a layer of ice. When the shot changes, the ice has disappeared altogether. (00:18:15)
Heart of Ice - S1-E3
Continuity mistake: When Batman breaks free of his ice handcuffs, the colors on his chest emblem are inverted. (00:15:30)
Heart of Ice - S1-E3
Continuity mistake: When Mr. Freeze's henchman Johnny gets his legs frozen, an ice "bridge" connects the gap between his feet. A few shots later and the connective ice is gone, making it look like he is wearing boots made of ice. (00:05:45 - 00:07:00)
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)
Continuity mistake: When we see Two-Face in full for the first time in the previous episode, his left eyebrow is white. In this episode, his left eyebrow is black.
Continuity mistake: When Harvey Dent is scarred in the previous episode, it's only the left side of his face and neck. In this episode and all others with Two-Face, his left hand is also scarred and discolored.
Continuity mistake: The mud stain on Harvey Dent's suit disappears when he assaults the henchman that kicked the mud on him and then reappears when he is walking away from the scene. (00:04:50 - 00:05:50)
Character mistake: When Batman uses his magic computer to analyze Langstrom/Man-Bat, lots of lovely gibberish appears on screen, with some notable typos such as "Bat membaaanes", "Sedum Chloride D", "Mammac higi Index Gat harmal nomeaace at padiotion." Your guess is as good as mine for the last one. (00:21:10)
Character mistake: When Batman gets the information from the computer just in time to wrap up the episode neatly, the computer misspells the word "Professor." (00:19:40)
Continuity mistake: In the bat nerds laboratory, Bruce Wayne shows them the tape; his hand suddenly turns between shots. (00:12:55)
Continuity mistake: For one frame during the final battle with the werewolf, the logo on Batman's chest is black and white instead of black and yellow. (00:17:55)
Heart of Ice - S1-E3
Continuity mistake: In the wide shot of Batman hanging upside down in Mr. Freeze's lair, his face is blank and the bat outline from his chest emblem is missing. (00:14:10)
Continuity mistake: When Harvey Dent wakes up from his nightmare, his face is sweating profusely. A couple of shots later and there is no sweat to be seen on his face. (00:02:10)
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.