Revealing mistake: In the scene close to the end when the turtles are fighting the foot soldiers on the fire escape (directly before the scene where Mikey ducks under the foot soldier's axe), Raphael thumps a foot soldier on top of his head and then rams the soldier's head into the stairwell. The only problem is the soldier's head misses the stair by about a mile. The sound effect of impact is there and the soldier even holds his head as if he were hurt.
Revealing mistake: In the scene where Raphael is thrown through April's skylight after being attacked by a few dozen members of the Foot, the other Turtles and April check on him. As Donatello and Leonardo look him over, they lean on his shell, bending it with great ease even though a turtle's shell is supposed to be almost as hard as rock.
Audio problem: At the climax of the film when the Turtles are about to face Shredder for the first time, Raphael says, "I got him" and then runs to fight. When Raphael falls to the ground, his pair of Sai hit the ground but make no noise. There should be a metallic sound against the rooftop.
Revealing mistake: A lot of times in the movie, when the turtles' mouths are wide open, you can see the cast. An example of this is the scene when Raphael wakes up in the bath tub and Leo hugs him. When Donatello tilts his head back and laughs you can see eyes, a nose, and a mouth within the mouth of the costume.
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Continuity mistake: When Splinter is holding the Shredder over the building's ledge, the cord on Mike's discarded nunchaku changes position in relation to the spear's point. It moves closer to the point for a single shot, then moves back to its original position in the subsequent shots.
Continuity mistake: After Raphael has a fit and walks out of April's apartment, he goes up on the roof to vent his anger for a while. Pretty soon, Casey spots him from another rooftop several buildings away while tuning a radio. As Casey looks out at Raphael's faint image in the distance, pay close attention to the background as well as the white building Raphael is standing on (you can notice the Empire State Building among other landmarks). When Casey grabs some binoculars to take a better look, Raphael can be seen resting on a brick ledge. In the following shot Raphael looks out at the city towards Casey's vantage point, but from Raphael's position, the Empire State Building - as well as the white building where he was venting just moments before - are visible on the skyline. Somehow Raphael has managed to relocate to another building's rooftop between shots.
Continuity mistake: When Splinter flips Shredder over the edge of the building, Shredder pulls a knife and throws it. When Splinter catches the knife, he catches it with his right hand; in the next shot, Splinter's right hand releases the nunchaku and Shredder falls. The shots are out of order.
Audio problem: After Leo's failed attempt of fighting Shredder for the first time, Mikey and Donnie then play Rock, Paper, Scissors to see who fights Shredder next. It then cuts to Casey talking to "The Foot" where you can hear Mikey fighting Shredder. The next scene shows all four Turtles in a huddle regrouping, breathing heavy, and out of breath. According to what we heard, Donatello shouldn't be out of breath as he has not fought Shredder for the first time yet.
Continuity mistake: During a fight sequence between the Foot Clan and three of the turtles in April's apartment, keep in mind that April is still tending to an injured Raphael in the living room. But as the floor collapses into an antiques store below, you'll be able to spot her on the lower left side of the screen shielding her eyes from debris - instantly in the store before anyone else (except for Master Tatsu and several other Foot members, who were waiting there to begin with).
Continuity mistake: When Danny is admiring April's turtle sketches, he asks if he can keep one of them. His right hand holds onto a sketch of Leo at first, but in the following shot his right hand isn't gripping the sketch at all. Instead, he's seen reaching out to pick it up from the couch.
Revealing mistake: After Splinter releases the nunchaku from Shredder's spear, he falls from the building's ledge. But notice how Shredder stops abruptly right before the series of shots which show him taking the long fall into a trash compactor, indicating that he actually lands on a mat.