As a Turtle, Michelangelo dons clothing for his transformation into a human, but afterwards his clothing vanishes and reappears depending on whether he's a Turtle or a human. Similarly, his mask and wrist/elbow/knee bands vanish when he turns into a human and reappear when he transforms back.
When the three Turtles find the Michelangelo lookalike teenager, he was wearing the same Hawaiian polo shirt, blue shorts and red shoes as Michelangelo in his human teenager form, but in a later scene, he is now wearing a plain light purple polo shirt and white shoes.
When the three Turtles find the teenager who looks like Michelangelo in his human form, he wasn't carrying his skateboard, but in a later scene, his skateboard suddenly appears under his arm.
When Leonardo says "it's unstable" he speaks in Donatello's voice.