We don't live in an ideal world. Humans aren't able to measure exact quantities of substances; not even specially designed machines are completely reliable. In any reaction, there will always be one reactant that you should have added more of. However, even if you were a superhuman and could measure perfectly, you still would be unsure of the results of your reaction.
Chemical reactions never completely consume reactants. You will always only get a percentage of the product you think you will. Chemical reactions are not just equations that you write down on paper; they are living works of nature that produce heat and energy.