This week's Torah portion, Chukat, is where Moshe & Aaron commit a sin so grave that they are banned from entering the Land of Israel. Let's look at the two lines where this happens (Numbers, 20/10-11):
(10) Moshe and Aaron gathered the congregation before the rock and he said to them, "Listen now, O rebels, shall we bring forth water for you from this rock?" (11) Then Moshe raised his arm and struck the rock with his staff twice; abundant water came forth and the assembly and their animals drank.
So what exactly was his sin? Considering it's been something commentators have delved into - let's look at four possibilities (pointers courtesy of The Stone Edition Chumash) that resulted in the aforementioned punishment:
1) Rashi's explanation of Moshe's sin is straight forward: Moshe was commanded to speak to the rock ("Take the staff and gather together the assembly ... speak to the rock before their eyes and it shall give its waters," Numbers 20/8) and instead of speaking, he struck the rock.
2) Rambam explains that Moshe's sin was getting angry with the people as he tried to quiet them, "Listen now, O rebels." Why was Moshe's angry reaction to the people such a sin? Rambam states in Shemoneh Perakim that Am Yisrael saw Moshe's words as a reflection of Hashem's will and hence, Moshe's angry reaction in fact showed that Hashem was angry with his people. As we don't find anything in the chapter, be it stated or hinted, that Hashem was angry with the request, we can assume that Moshe mislead his people into unnecessary worry and angst. This can be a vital lesson to us - We should try and contemplate how others will react to our actions before acting them out.
3) Rabbeinu Chananel and Ramban key in on "Shall we bring forth water..." as being Moshe's sin. Moshe's statement implies that he and Aaron are responsible for bringing water out of the rock, not Hashem. This could be the reason Hashem reprimanded the brothers by saying, "Because you did not ... sanctify Me in the eyes of the Children," (Numbers, 20/12). Moshe should have said, "Shall Hashem bring forth water" ...
4) Abarbanel, despite agreeing with Rashi about the sin, claims that the reason for the severe punishment was to finally punish Aaron (golden calf) & Moshe (sending the spies) for their past sins gone unpunished. Like the rest of Am Yisrael, Moshe and Aaron were not given the privilege of conquering the Land. When great men like Moshe and Aaron never walked on this land, it makes me appreciate the fact I get to walk around Eretz Yisrael daily even more.
So there you have it ... There are other takes on this sin, so if you want to add another explanation, feel free to do so in the comment section.