Revealing mistake: When Bob and Ben prepare to gas the ant nest, an ant appears below them. But its legs move too slowly, compared to the speed in which it appears; it rather looks like it is driving across the ground on (hidden) wheels.
Revealing mistake: When the heroine sees the creature emerge from the cave and strangle the man she was with, she hams up her acting in the worst, most blatant way - she throws herself to the ground, screaming. She didn't even make it seem like she tripped or anything.