A minor transgression is not necessarily one that is without effect.
On the contrary, the more righteous one is, the more sensitive one becomes to sin.
The Talmud tells how the strap of Rav Huna’s tefillin once got twisted so that the black side, which should be exposed, was’ turned inward.
To rectify this “sin,” Rav Huna fasted forty fasts.
This was an extremely minor matter, done inadvertently, and from which he certainly derived no physical or spiritual pleasure or gain.
Yet to Rav Huna, this minute flaw in the manner of his fulfillment of a mitzvah was nothing less than devastating
–Rabbi Adin Steinsaltz
From Opening the Tanya