In the beginning, after they save April and are just chillin' in the sewer, one of the turtles, I think it's Michaelangelo, is skateboarding. But they only show from the legs down. There is a moment where the hand of the skateboarder appears on the screen and it is a human hand! Not a turtle's hand!
Strange, I looked for this on the DVD, but I couldn't see it. So I watched the original movie trailer on the DVD and yes, you can see a human hand for at least two seconds. But I couldnt see it in the movie, I couldn't find it. And yes, its Donatello on the skate board.
I just watched the VHS version of the movie (without prior knowledge of this slipup), and I caught that error too. You can see a white-sleeved arm sweep past the top of the screen for a second or two. This is easily explained (the actor in the Donatello suit probably wasn't a very good skateboarder, so they used a guy who knew what he was doing and just gave him 'turtle legs'), but wasn't caught. Or maybe it was, and was just too late to fix.