Revealing mistake: In the junkyard, Leo and Raph grab a long pole and plough through several Foot Ninjas with it. It is painfully obvious the ninjas react to the pole before it ever hits them. Especially the guy on the ground. (00:53:20)
Revealing mistake: When Raphael and Keno are in the junkyard and the fight breaks out, some of the weapons bend and twist. The staves are supposed to be solid wood. (00:45:50)
Revealing mistake: There's one scene where they are surrounded by the foot. Leonardo reaches behind him for his katanas, but he doesn't unsheath them. If you look at his elbows, you can see gaps in the pads where the actor in the suit hasn't fully fastened the gloves to the rest of the outfit. (01:05:40)
Revealing mistake: In the first fight scene of the film, Keno sweep-kicks one of the bad guys. The man falls over without ever being touched. (00:03:05)
Revealing mistake: In the part where Rahzar grabs Mikey by the arms and begins swinging him around at the concert, look at Mikey's shoulders. You can see the material of the actor's body suit folding at the shoulders as it is bent further than it was designed for. (01:08:15)
Revealing mistake: When Donatello is messing around with the puppet, in the background the stuntmen who are fighting Leo and Mike are doing incredibly sloppy choreographies, or lack thereof, in particular those in the middle (around the doorframe) who literally run on the spot without doing anything. (00:05:45)
Revealing mistake: When Michaelangelo throws the flask to knock away the canister of ooze, the ooze flies away before the flask reaches it. (00:25:50)
Revealing mistake: Once Shredder yells "Let the games begin", there is a cut on, so to say, the bleachers with the various Foot clansmember cheering and waving their weapons around enthusiastically. Their enthusiasm is too much and/or the weapons are too fake, because you can see their staves bending and jiggling like toys. (01:06:40)
Revealing mistake: Tokka tosses Leonard like a bowling ball; the Foot ninjas tumble and fall even when they are miles away from making any contact. (00:53:00)
Revealing mistake: In the rooftop scene with Splinter and the canister of Ooze, the blurry fake backdrop is not in the least convincing. (00:21:30)
Revealing mistake: While Donatello is doing the "Ninja Cowboy" gag, in the background Michelangelo slips away from a dogpile and lets the goons beat each other up. It's meant to be a mere background choreography and it shows, since the stuntman on top is "pummeling" the others with the slowest fake punches. (00:05:05)
Revealing mistake: During the opening scene a clear divide between head and body of the Turtles suits is particularly obvious in a few sequences, such as when Donatello hogties a baddie and shouts "Ninja Cowboy!" raising his head, or when he plays dummy bouncing left and right. (00:05:20)
Chosen answer: He had moved away - he says in the third movie. We are never told as to where it was he moved to though.