Towards the end the trapped group of people having almost made it to their car when they see a worm in the way. This is when the slip-up occurs. But brings out his very large, anti-tank rifles and shoots the worm. But as the shot goes off, the director wanted the viewer to think that this gun is so powerful its shock wave would blow out a window. But look at the window as the gun goes off and you will see a crew member stand up and hit the window with a 2x4 (a piece of wood). This slip-up is obvious if your looking at the window in the background and not the gun, as the director wanted you to.