You can also get negative amounts due to rounding in the label.

For instance, let's use a hypothetical class of food that actually follows the 9-4-4 rule. If the product actually has 10.5 grams of fat and 7.5 grams of protein the total calories would be 124.5. So the manufacturer might label this as 11g of fat and 8g of protein and 120 calories. Using those numbers the carb calculator would come up with a carb count of -2.75g.

Because of the rounding in the label data, I think the carb calculator is really only useful in identifying foods that truly have hidden carbs - ingredients that the manufacurer is intentionally leaving out of the carb count. If the calculator says that the food has a couple more or a couple fewer carbs than the label, assume the label is correct or take the count from which ever one is higher. The impact will be minimal either way. But if the carb calculator is saying that the food has tens of carbs more than the label says, then you know that it has truly hidden carbs in it and must consider whether to eat it much more carefully.