Seeing the topic, everything done right but the result is wrong... The first thing that struck my mind was chemistry... Knowing the whole processes arrangement doesn't really work, getting the right proportions is also important.
Take titration for example, you have a solution to mix and a set colour you need to meet. An extra drop of an indicator can totally alter your result and mess up the experiment. Even the same experiment can give you several results because of a little miscalculation or something...
Yes, yes...