On the level: Assault On The Control Room, take any vehicle and move through the level until you get to the part where at the bottom of the screen it says: "Rolling Thunder". Instead of going forward and look back at the Marines (if you helped them survive through the level until this part) that were chasing after you on foot. They will be frozen in running position. This happens because you took a vehicle through the checkpoint. I recommend not killing them, because if you go through the level normally and come back, they will be unfrozen.