Mistakes to avoid
Represent a technical system according to the law of integrality of parts

For example, you can get the systemic level wrong. For example, glasses seem like a nice tool for a study, but can be ambiguous if you don't separate the "lens" tool from the "frame" tool, both of which take you to different UPFs.

In cases where the UPF is ambiguous, you may have the wrong object. For example, what is the UPF of a hand dryer? There are two opposing alternatives: "the hand dryer dries hands" or "the hand dryer removes water". In the first case, the tool is "hands", in the other it's "water". In such a case, by applying the rule of eliminating the surface of the earth from the object, we arrive at the following conclusion: an earth without water renders the hand dryer useless, whereas an earth where men would have no hands could possibly find another use for the hand dryer (e.g. drying the arms or the face).