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)
Continuity mistake: When a policeman warns Gordon that the SWAT team is in place, there's nothing to the right of the police car; in that spot there was another patrol car, and a policeman with his gun out. He ducked when the villains started shooting, but he couldn't have gone away without any cover. (00:02:55)
Continuity mistake: The pothole with the mud that Rupert Thorne's henchman kicks at Harvey Dent appears from nowhere. When the henchman is shown kicking the mud, the pothole is located between the police officer behind the henchman's right shoulder and Harvey's assistant Carlos on the henchman's left. However, in the previous shot, the ground shown between the police officer and Carlos contains no pothole. (00:04:35)
Continuity mistake: Carlos' suspenders are not drawn in the shot before he starts clapping - and the ruffian is getting dragged to the police van. (00:04:45)
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)
Continuity mistake: Harvey Dent's mud stain is different when he looks in disbelief after Thorne's goon says he'll chew and spit him. He's got a stain on his lapel that was never there before and is gone later. (00:04:55)
Continuity mistake: When Harvey Dent is walking away from the crime scene, Commissioner Gordon is on his right when next to the police van. Later when Rupert Thorne is watching the news report, Harvey is shown walking by the police van by himself. (00:05:40 - 00:07:00)
Continuity mistake: As evidenced by the news report that Thorne is watching, Carlos and a photographer by him are looking at Harvey's aggression from two different places practically at once. (00:06:10)
Continuity mistake: When "Big Bad Harv" is summoned by the shrink and grabs the coin he tossed, the cuff of the shirt is red instead of white. (00:10:50)
Continuity mistake: Once Big Bad Harv identifies himself and tosses Thorne against his goons, the dossier is gone from the desk as he poses all menacing, and is back there later. (00:18:10)
Continuity mistake: The explosion that scars Harvey Dent and creates Two-Face is shown to have scarred and colored him light blue on the entire left half side of his face and neck. However, when he is first shown in the hospital, the parts of the left side of Harvey's face that aren't covered in bandages are still normal, as is his entire neck. (00:19:35)
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.