Revealing mistake: When Joe drives Camille on his bike, for a brief moment if you look closely a stuntman is driving for him.
Revealing mistake: Toward the end of the movie when Van Damme is on the motorcycle watch the pull back shots. You can see the stuntman in place of Van Damme. In addition, the stuntman is clean shaven in some shots where Van Damme looks like his beard is growing in. It's very noticeable, especially when he goes down the mountain. You can even see the ramp set up as he first goes down.
Revealing mistake: The Turtles are preparing their time travel. Splinter tells them "The scepter will be in the temple, which means..." Pay close attention here, because now the rest of the sentence is obtained dubbing the line over a rewind of the previous footage. (00:21:50)
Revealing mistake: When Michelle Yeoh is about to hit the basket on the fire truck you can see her safety wire.
Revealing mistake: During the tournament we see one fighter, a woman in pink spandex, fight Catya Sassoon twice. It's obvious due to the fact that Catya's wearing different clothes during each match.
Revealing mistake: When Emilio is breaking into the cookie factory, he finds a key under a welcome mat. He then opens the door before fully unlocking it with the key.