Remember that feeling of "it's just not fair"?   Whatever it may have been that Moses did wrong - his punishment seems much too harsh.

Crime and Punishment

"Because you did not trust Me enough to sanctify Me in the eyes of the children of Israel, therefore you will not lead them into the land."   (Num 20:12) 

Moses and Aaron could have done something great to 'sanctify YHVH in the eyes of Israel'.  However, the Torah never tells us precisely what they did wrong.

Following Instructions

Let's compare what Moses was told to do (Num 20:8) with what he actually did.  (Num 20:9-11)

YHVH gave Moses five commands to execute.

"YHVH spoke to Moses saying: take the staff, and gather the congregation together, you and Aaron your brother, and speak to the rock before their eyes that it should give water, and take out for them water from the rock, and give drink to the people and their animals."   (Num 20:7-11)

Command #1 - "Take the staff."  

Moses' Execution:  "And Moses took the staff from before YHVH, as He had commanded him."   

Nothing seems to be wrong here, the verse confirming "as YHVH commanded him."

Command #2 - "Gather the congregation."   

Moses' Execution:  "Moses and Aaron gathered the congregation together in front of the rock." 

Here, too, no sin appears to have been committed.

Command #3 - "Speak to the rock that it should give water." 

Moses' Execution:  "Moses said to them, listen here you rebellious people, is it possible that we can take water from this rock?"   

Here, YHVH commanded Moses to speak to the rock, but Moses never did.

His rebuke was indeed warranted, nevertheless, he should have still followed YHVH's command to speak to the rock and not about it!  

Command #4 - "Take out for them water from the rock."  

Moses' Execution:  "Moses lifted his hand and hit the rock with his staff two times, then much water came out." 

Command #5 - "Give drink to the people and their animals."  

Moses' Execution:  "The people and their animals drank."   

Clearly, Moses did nothing wrong in this final stage.  No matter how we explain Moses' sin, the question remains:

why was his punishment so severe?

It appears that Moses didn't really do anything that bad at all.  Just "talking about the Rock" instead of "to the Rock".  Is it possible that Moses and Aaron were actually punished for earlier sins - Moses for sending the spies and Aaron for the Golden Calf.  Was this just a kind of 'cover-up' to isolate Moses and Aaron's wrongdoing from that of the nation?

Let us look at the opening six verses for the answer.  (Num 20:1-6)  

"The children of Israel arrived at the desert of Zin, but there was not enough water for the people, and they gathered against Moses and Aaron.  They argued with Moses saying:
It would have been better had we died with our brethren before YHVH.  Why did you bring us to this desert to die  
and why did you take us out of Egypt to bring us to this terrible place.  There are no fruits here and there is no water to drink."  (Num 20:1-5)

How did Moses and Aaron respond to these complaints?  
Did they defend YHVH?  Did they offer the people encouragement?

"And Moses and Aaron went from the congregation and came to the Tabernacle, and they fell on their faces."  (Num 20:6) 

Why didn't Moses take a stand like he did at Rephidim, when Israel complained?  There, his immediate response was:

"Why are you arguing with me, why are you testing YHVH."  (Ex 17:2) 

Moses challenged their complaints and condemned their criticism as a reflection of their lack of faith.  Only afterward, when the people continue to complain, he cried out to YHVH.

This time his reaction was quite different.  Instead of confronting their complaints, he immediately ran away to the tabernacle, as if afraid of a confrontation with the people.  Was 'running away' the proper reaction for a leader?  Should he not have assured the people that YHVH will indeed take care of their needs?  Should he not have challenged the people's assertion that "it would have been better had they remained in Egypt?"

Already at this early stage, Moses and Aaron failed as national leaders by not sanctifying YHVH's name when the opportunity arose.  This may be precisely what YHVH meant when He rebuked them.

"Because you did not trust Me enough to sanctify Me in the eyes of Israel."   (Num 20:12)

Whose Staff Was It?

What about hitting the rock?  (Num 20:7-11) 

"And YHVH spoke to Moses saying: take the staff and gather the people."  (Num 20:8)

It is commonly assumed that Moses was to take his own staff, the very same staff with which he brought the plagues, split the sea and produced water from the rock at Horeb.

However, this cannot be.  The verse states explicitly:

"And Moses took the staff from before YHVH, as YHVH had commanded him."  (Num 20:9) 

This referred to the rod in front of the Ark, the Ark of the Covenant, located in the Holiest part of the Tabernacle. Moses did not keep his own personal staff in the Holy Place, so whose staff was it?  It was Aaron's special staff.

"Return the staff of Aaron in front of the Tablets of Testimony for safe keeping, in order that it be a sign for any rebellious group so that they will stop complaining and not die."  (Num 17:5,10)

Moses was told that the next time Israel complained or rebelled, he should take Aaron's staff from the tabernacle to remind them of what happened to rebellious Korah.

This, the waters of Meribah, was the very next time Israel complained, and YHVH had to tell Moses to do what he should have done on his own.  (Num 20:8)  That's why Moses told the congregation "listen you rebellious ones."  (Num 20:10)

Considering that YHVH had instructed Moses to take the "rod of Aaron", set aside as a "token against rebels", it was appropriate that he rebuke the people accordingly.  

Two Staffs At Meribah

1) The staff of Aaron was used to remind the people of their rebellion, during this entire event.
2) The staff of Moses was used to hit the rock to bring forth water.

Why did Moses hit the rock?  At Horeb, YHVH never told Moses how many times to hit the rock.  Even if the extra blow involve wrongdoing, could this 'slight' transgression warrant such a severe punishment?

Moses and Aaron were punished for not having sanctified YHVH's Name earlier - not for hitting the Rock.

Punishment or Demotion?

It is commonly assumed that Moses' and Aaron's punishment was that they were denied entry into the Promised Land.  However, this is imprecise.  YHVH told Moses, "Because you did not trust Me enough to sanctify Me, therefore you shall not lead this nation into the land that I promised them."  (Num 20:12)

This suggests that Moses and Aaron have failed as leaders and for that reason they would not lead Israel into the Promised Land.  They were punished not as individuals, for 'technical' flaws in their execution of YHVH's command, but as national leaders, for a flaw in the nature of their leadership.

YHVH expected that Moses and Aaron would use the rebellion at Meribah as an occasion to rebuke Israel and thus sanctify His Name, but they failed to do so.

Failure in leadership does not require the leader to have done something 'wrong' or commit a sin.  A leader, as the term implies, must lead the people, be an example and take the initiative.  As individuals, Moses and Aaron did not sin at Meribah but as leaders they failed YHVH and the people.

"Because you did not have faith in Me in the eyes of Israel."  (Num 20:12)

Although YHVH immediately gave Moses and Aaron detailed guidelines on how to deal with the situation, it was already too late.  As soon as the incident was over, YHVH informed them that their days as leaders were numbered.  Before Israel could begin their conquest of the land of Canaan, a new leader would have to be appointed.

Later, in the book of Deuteronomy, when Moses begged to be allowed to see the land, he did not ask to lead, only to see for himself.  (Deut 3:23-26)

Had this been the only incident where Moses' and Aaron's leadership faltered, the consequences would likely not have been so harsh.  However, this leadership flaw had already surfaced numerous times before. 

Moses himself admitted his inability to lead the people.  (Num 11:11-15)

Miriam, Moses' own sister, expressed disapproval over his leadership. (Num 12:1-3) 

Caleb and Joshua had to assume the role of leaders to calm the nation in the 'spies' incident.  (Num 14:5)

New leadership was required to meet the challenges of taking Israel into their Promised Land and Joshua was chosen by YHVH to be Israel's next leader.