Life is messy. When the ideal system of halakhah is applied to real life situations, with all their complexities, disputes often emerge. Debates often spill over from one area to another, when a strict ruling here leads directly to a lenient ruling elsewhere. Requiring a large minimum amount for eating matzah on Pesach also allows for avoiding reciting the long grace after meals when less than that amount of bread is eaten on other days.
The adoption of conflicting lenient rulings — acting permissively in both situations (e.g. eating a small amount of matzah but only reciting grace after eating a large amount of bread) — is declared wicked by the Talmud (Rosh Hashanah 14a-b), due to insufficient piety. Acting stringently in both situations, eating the large amount of matzah but reciting grace after the small amount of bread, according to the Talmud, is foolish. “The fool walks in the dark” (Eccl. 2:14) speaks of such a person.
However, a person can adopt contradictory stringent rulings for more than one reason. Are all defined as foolish? Rashi (Eruvin 7a sv. de-sasran) writes that the problem is when someone knowingly adopts different and inconsistent strict positions. This attitude of excessive piety is foolish and undesirable. Tosafos (Rosh Hashanah 14b sv. ha-kesil) state that even someone who adopts one view as law and then volunteers to follow the other view and act inconsistently strict also falls under the stricture of excessive piety. That is too much, Tosafos seem to be saying. Find an approach and stick with it.
However, Rashi elsewhere (Rosh Hashanah 14b sv. ve-ha-rotzeh) describes the person’s foolishness as an inability to recognize the contradiction. Arukh La-Ner (ad loc.) connects this explanation with that of the Ritva (Eruvin 7a) — he should have learned enough to understand when to be strict and when to be lenient. According to the Ritva and this view in Rashi, the foolishness is ignorant piety.
The difference between excessive and ignorant piety is not just about level of knowledge, that even a learned scholar can be excessively pious. It is about which intentions are desirous to God. According to the first view above from Rashi, piety requires moderation. In life we must avoid extremes, as the Rambam (Mishneh Torah, Hilkhos Dei’os 1:4) famously advocates. Even careful mitzvah observance has a limit: inconsistency. According to the Ritva and the second view above from Rashi, excessive piety plays a role in Jewish life but it must be adopted with scholarly care. Perhaps we can suggest that it is a level of religiosity reserved specifically for scholars, those who have mastered the laws and can avoid foolish inconsistencies. Only after climbing the ladder of Talmudic mastery are you prepared for the mantle of piety.