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 Grace is crying over Harvey, she is holding a picture of him with her left hand while stroking the frame with her right hand. When the shot changes, she is suddenly gripping the frame with both hands. (00:06:15)
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: There's a marked cone of light on the door that leads to Harvey's room inside club Wild Deuce; it disappears after the opening shot. (00:15:05)
Continuity mistake: When Batman drives his two-wheeled vehicle out of the Batcave, there are no numbers on the concrete as they appeared in 1x8.
Continuity mistake: Grace puts down the coffee mug. When "Detective Leopold" shows her in close-up the beeper, the cup has turned around, with the handle where it would have been visible earlier (but it was not). (00:06:50)
Continuity mistake: When Bruce tears away from a clip a picture of Harvey, the clip was part of a clipboard with other papers underneath that, that are now gone. (00:06:00)
Continuity mistake: Thorne bangs angrily his fist on the newspaper with the article about the robbery. The letter opener bounces and stops, but in the next shot it's gone. (00:03:55)
Continuity mistake: When the twins enter the bookie's cashier spot (through an unlocked door), the money is scattered differently on the counter between shots. (00:03:00)
Continuity mistake: When the chandelier falls on top of Rupert Thorne, the tommy gun he was holding falls into the bright half of Two-Face's hideout. Later when Two-Face grabs the gun, it is in the dark half of the hideout. (00:19:15 - 00:20:10)
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.