Revealing mistake: When Juney and that other guy are singing and strumming the guitars, it's very obvious that Juney isn't strumming. (00:14:25)
Revealing mistake: When the guy who is supposed to be Jumper lands on Prince Caspian's sword, you can tell he's a dummy (just watch his head flop around). (00:32:15)
Revealing mistake: When the mammoth stomps on the caveman at the beginning of the movie, you can tell it doesn't even touch him because you can see the actual footprint about a yard across from him.
Revealing mistake: When Juney is chugging the Sunny D, you can see most of it leak out on the side of the bottle so she didn't have to drink it that much. (00:22:20)
Chosen answer: Apollo 440 - Stop The Rock.