There is a special, basic instruction that one can derive from accountancy: that a discrepancy of even a penny indicates a basic error in the bookkeeping as significant as if it were a substantial amount. This is not the case in ordinary business transactions where a discrepancy of a penny would be of no consequence. This helps us to better understand the imperative: “Be as careful with a small mitzvah as with a stringent one.” It is difficult at first glance to understand how one can compare a small mitzvah with a stringent one, especially as the Torah itself terms one small and the other stringent. But in light of the above, if one truly makes an honest reckoning, he must realize that all mitzvos come from the same Source, and each one of them sanctifies us. Therefore, it can be said that a transgression of even a “small” mitzvah is as deplorable as transgressing one that is stringent.

Sivan 9, 5739, taken from a translation at Chabad.org