Factual error: During the scorpion/ant fight scene, the ant and scorpion are close to the same size. In reality the scorpion would exceed the ant by many times. The ant wouldn't be able to fight it, and the scorpion probably would hardly notice it was around.

Factual error: From what I gathered, the machine compresses the empty space between matter, which in turn shrinks the object in its path. So the mass of the shrunken object is still the same, meaning the weight is also the same. This would mean that their weight on such a small area would create extreme pounds per square inch pressure on anything they step on, causing them to press through the wood, burrow into the ground, etc. They would probably need snowshoes to overcome this. Plus Rick Moranis could not sweep them away and put them out to trash, since the weight of them alone would easily exceed 400 pounds.