The Midrash Rabbah (19:12) explains Moshe’s sin as follows:

“It can be compared to two women who are punished by Beit Din. One kilkilah [i.e. she committed the grave sin of znut], and one ate pagei shvi’it [i.e. she committed the lesser sin of eating pagei shvi’it].

[The latter woman] said to them, “Please inform the world what I being punished for, lest it be said that kilkalti. [i.e. Please make sure that everyone knows that I am being punished for the lesser sin of eating pagei shvi’it so they do not chas vishalom think that I sinned gravely by committing znut.]

In other words, the Midrash equates Moshe’s sin of hitting the rock, rather than talking to it, to the sin of eating pagei shvi’it. Rav Neriah zt”l clarifies this comparison.

What is the problem with eating pagei shvi’it? During shemitah, unripe fruit are forbidden, because one is required to utilize the fruit’s maximum potential. Thus, one may only eat the shemitah fruits when they are ripe and juicy.

Moshe’s sin was that he did not utilize the complete miracle inherent in drawing water from the rock. If he had spoken to the rock, there would have been an incredible Kiddush Hashem. As Rashi notes (based on Chazal), Bnei Yisrael would have learned the following kal vichomer:

“Just like this rock, which can not speak and does not hear and does not require sustenance, fulfills Hashem’s words – all the more so must we.”

Thus, by hitting the rock rather than speaking to it, Moshe did not fully utilize the great potential and tremendous benefit involved in extracting water from the rock.

In this way, his sin was similar to eating pagei shvi’it. Both cases entail wasting many potential benefits.

May we always extract the maximum Kiddush Hashem from every situation.