Revealing mistake: When the group comes across the first ass-blaster, it hops out from behind the fence. Pay attention to its right leg when it jumps out and you'll see a rock on the ground just appear behind it. After it spots them, it stomps a few times. While it's stomping, pay close attention to the rock and you'll see it and some of the ground around it just disappear after a few frames. This rock also isn't visible in any other part of the scene.