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: 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 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: 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: 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)
Other mistake: When Two-Face is looking at the picture of Grace in his wallet, you can see he has a credit card that says Two Fa[ce]" on it. A bank wouldn't issue a credit card to someone under that kind of pseudonym, and it would be very foolish for a known criminal to use a credit card since any purchases can be tracked. Even if it was a company credit card for a dummy corporation, it would be equally foolish for Two-Face to have a dummy corporation named after himself. (00:08:05)
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)
Plot hole: When Two-Face and his men have broken into the new D.A.'s office to find dirt on Rupert Thorne, one of his men finds a file detailing Thorne's record of Swiss bank accounts, money laundering, blackmail and payoffs that Two-Face tried for years to subpoena when he was D.A. When Thorne finds out that Two-Face has the file, he states that he will be ruined if Two-Face gives the file to the police. If Two-Face tried unsuccessfully for years to subpoena the file, then in all likelihood that means Thorne paid off the right people to prevent the subpoena from happening, so Two-Face giving the file to the police shouldn't be a problem for him. Additionally, if giving the file to the police would be all that it would take to bring Thorne down, then the new D.A. could have already done so. If the new D.A. was also paid off by Thorne, then it wouldn't make sense for Thorne to allow the D.A. to keep the file since they could easily lose the file or even double-cross Thorne. (00:10:45 - 00:14:20)
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)
Audio problem: When Rupert Thorne is saying "That file is here all right," his mouth is closed for the first four words. (00:18:10)
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)
Other mistake: When Two-Face flips his coin in the air to decide whether or not to kill Rupert Thorne, Batman is surveying his surroundings, finding a box full of silver dollars and laboriously tossing the box into the air in order to make Two-Face lose sight of his coin. In the time it took Batman to do that, Two-Face's coin would have already landed back in his hand. (00:20:30)
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: When Batman drives his two-wheeled vehicle out of the Batcave, there are no numbers on the concrete as they appeared in 1x8.
I've Got Batman in My Basement - S1-E20
Continuity mistake: The talon cuts on Batman's cape change in space and position between shots; they are firmly on top of the shoulders at the end of the struggle, but they were lower during the actual fight. (00:02:20)
I've Got Batman in My Basement - S1-E20
Continuity mistake: Batman looks at the vulture after outwrestling it, and then, sprints to get his 'rang back. When he looks out he's got a cut on the suit, on the left, but when he fetches the weapon the torn part is gone. Cut away, and now the tear is back and doubled over to the other side. (00:02:55)
I've Got Batman in My Basement - S1-E20
Revealing mistake: The two kids grab a couple of items from Sherman's detective box, but the items never go away, a duplicate is spawned instead. (00:03:30)
I've Got Batman in My Basement - S1-E20
Revealing mistake: Sherman puts down the Jr. Detective box to try to get back the stuff the bullies grabbed. He sets the box on one of the steps, but the way it is drawn defies gravity. (00:03:40)
I've Got Batman in My Basement - S1-E20
Revealing mistake: The thief kicks away the canister bird seed was falling on...and the bird seed keeps landing in the same spot, which at this point is thin air. (00:05:10)
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.