I've Got Batman in My Basement - S1-E20
Character mistake: One of the books Sherman is getting his detective stuff from is called "Crime and It's Causes." It must not be a very good book. (00:11:15)
I've Got Batman in My Basement - S1-E20
Other mistake: The Batmobile moves because Roberta is pushing the gas pedal...but one of the first shots when we see the car move, shows Roberta rolling to the back of the cockpit because of the acceleration. The car does not slow down or anything, though. (00:09:35)
I've Got Batman in My Basement - S1-E20
Other mistake: Penguin and the goons take an impossibly long time to get to Batman once he's incapacitated by the gas and is semi-unconscious at the railing. (00:07:40)
I've Got Batman in My Basement - S1-E20
Other mistake: Sherman touches by mistake the "OFF" button...which turns the conveyor belt on (there is a separate ON button...for some reason). Not just that; the buttons also look completely different between shots; first they are circular buttons with colored lights on top, then they are nondescript small levers. (00:06:55)
I've Got Batman in My Basement - S1-E20
Other mistake: The kids manage to drive away with the Batmobile simply operating the gas pedal, but they show that the gear is manual and it has three pedals.
Other mistake: The Stromwells are hugging in front of the office building, but that was the starting point 5 minutes before, there has been a long chase since. (00:21:15)
Other mistake: Batman leaps off a tall building carrying Stromwell and lands on one foot without breaking the fall. (00:12:25)
Other mistake: After the level crossing, the back of Stromwell's car is missing entirely the fender or any sort of lights. It's a close-up, even. (00:05:45)
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)
Be A Clown - S1-E11
Other mistake: The cards that Jekko the Jokester throws at Batman have in one shot ("Now it's my deal") numbers that do not match at all the symbols. (00:13:40)
Be A Clown - S1-E11
Other mistake: When Jordan comes out of the truck, something went wrong with perspective and/or proportions, because with his leg bent he is practically as tall as the truck door, and he's a child.
Plot hole: Alfred manages to find the bad guys because not only they grabbed the supposedly homeless guy off the street (Bruce in disguise), but also somehow brought his car to the scrapyard. If they knew it was his, they would have known something was off (a hobo with a car?) and they would have not got rid of it after more than a day. If they didn't, and they just towed out a random car, Alfred had extraordinary luck - nothing in fact would lead to believe that the truck was even related to the car.
Other mistake: Bruce remembers who he is, which somehow gives him also the strength to bend metal (!) and escape. A new prisoner is carried in the camp by the guards, but...he's not new at all. He was already there the first day, standing in the group of prisoners that was eating. And the guy who escorts him is the same that less than a second later spots Bruce from the watchtower and blows the whistle. (00:06:20 - 00:14:30)
Plot hole: The two agents split; the rookie goes down an alley, chasing the two henchmen, who reached their car and chase him back down the alley almost running him down. Batman saves him though, destroys the car and quickly interrogates a suspect. Somehow though he also manages to save Montoya, who went inside the warehouse right away as they split the warehouse. The timing does not work.
Other mistake: The kids need sunglasses to be in daylight, since they have been segregated into the darkness of the sewers for so long. For the happy ending though, only the first one has a moment (very brief, considering that Frog still needed them at Bruce's) of squinting, eye-rubbing and discomfort. The others are instantly fine already the moment they are pulled out of the manhole. (00:21:35)
Character mistake: Poison Ivy tells her beloved plant "I won't clip off any more of your beautiful petals." She clips off the whole flower, not really "petals." (00:14:35)
Character mistake: The French spoken by the waiter of the cafe is phony in a pretty atrocious way for a supposed high class restaurant. Can't even pronounce "monsieur" right, it's merely a tacked-on accent. (00:04:10)
Other mistake: Batman's lungs have plot armor; he has all the time to put his mask on even after exposure to the gas, and he reacts to it coughing at first. Nobody else in the episode reacts with a cough. (00:07:55)
Character mistake: Batman's computer warns him about the "air born gas emanating from river front district." It's airborne, and riverfront. (00:07:10)
Christmas With the Joker - S1-E38
Other mistake: Robin is trying to change channel but is unsuccessful, because the Joker hijacked all transmissions. Perhaps also because the remote control he has in hand has just one button. (00:04:40)
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.