Heart of Ice - S1-E3
Other mistake: When Batman is watching the security footage of Victor Fries' accident, at first Fries is addressing the camera directly which follows him and pans according to his movements and Ferris Boyle interrupting him, which would suggest a second person was in the room with a handheld camera. But after Ferris Boyle enters the room and orders Fries' experiment terminated, there are several cuts from various angles in the room suggesting surveillance cameras. The surveillance style footage also often conveniently cuts to the face of whichever person happens to be speaking or will cut to a certain angle in order to put emphasis on the action, all of which would require multiple mounted cameras and purposeful editing by either Boyle or one of his subordinates, which would be completely illogical since they don't want anyone to see the footage in the first place. (00:12:20 - 00:13:55)
Other mistake: Melissa Manchester and Neil Ross voice minor characters in this episode. The end credits list them like this though; "Additional Voices - Melissa Manchester as Neil Ross." It should have been "And", of course. (00:21:55)
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)
Other mistake: The eyebrow on Two-Face's left side is completely illogical. He wouldn't suddenly have an eyebrow that's bigger and bushier than his other eyebrow, and if anything the explosion that created Two-Face would have singed off that eyebrow. The eyebrow is there when the doctor removes Harvey Dent's bandages, so it's not like he put a fake one on for cosmetic reasons, nor would it make sense for the doctor to put a fake one on ahead of his face being surgically repaired. (00:21:15)
Heart of Ice - S1-E3
Other mistake: Batman uses a thermos full of chicken noodle soup to defeat Mr. Freeze, and moments later gives Summer Gleason a security tape that incriminates Ferris Boyle. When Batman pulls out both of these aforementioned items, there is a corresponding clicking sound that suggests he pulls them from his utility belt, but both items are far too large to fit inside the belt. They also could not have been simply been hanging from his belt, as Mr. Freeze had possession of Batman's belt a few minutes prior and they were not there. (00:20:05 - 00:20:45)
Christmas With the Joker - S1-E38
Other mistake: Batman and Robin are walking from the elevator to the Batmobile. Due to a coloring error, Robin's whole chest is green instead of red. (00:02:15)
Other mistake: At the beginning of the fundraiser for the university, people drop in a basket cash and cheques. All the bank cheques though have the sum written in the wrong format; for instance "five thousand dollars" is written "5.000.000." (00:12:30)
Christmas With the Joker - S1-E38
Other mistake: The train falls off the cliff, explodes, but during the explosion we see bounce off the flames...the train, perfectly intact, even with the lights on. (00:11:20)
Christmas With the Joker - S1-E38
Other mistake: Joker hosts the show in front of an audience of plywood mock-up figurines. In the wider views we see next to Gordon... Alfred. That would require Joker to know Batman's secret identity. (00:06:45)
Other mistake: When Batman listens to the tape left behind by the security guard (and the police's incredibly sloppy investigation) he presses play and not only the tape is at the right point without him having to rewind, but the recording is much different from the actual events and gives him a convenient abridged version. (00:08:00)
Other mistake: Batman is reading the front page of the newspaper, but in the drawings that follow, the paper is right-most, it opens to the left of the page. Gotham is not in Japan. (00:04:50)
Other mistake: On the police blimp, the scanner/radar screen in close-up has a single white circle, but in the wider angle it's two black circles, which are even hand-drawn and thus not perfect circles. (00:01:30)
Other mistake: Two Face's lips on the left side are a shade of pink instead of dark red and his coin's all rough looking on the edge.
Heart of Ice - S1-E3
Other mistake: When the children interrupt Summer Gleason's news report to play in the snow, the two police officers on the right are completely motionless for the entire 15-second shot. (00:02:10)
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)
Other mistake: Several shots during Harvey Dent's fundraiser show characters in the crowd never blinking or moving a muscle. (00:07:10)
Christmas With the Joker - S1-E38
Other mistake: Once Joker's back in his Arkham cell he should be wearing prison pants again like he was at the beginning of the episode. Yet he's still in his purple pants.
I've Got Batman in My Basement - S1-E20
Other mistake: Toss a brick into a pile of empty boxes, and they won't fly around like it is shown. The heavy brick will sink into whatever hits and possibly other boxes will be affected; here the brick does not deform the box it hits and every single cardboard acts like bouncy balls. (00:13:30)
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)
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.