Bemidbar 20-7: And G-d spoke to Moshe saying, "Take the rod and gather the congregation, you and Aaron your brother, and you shall speak to the rock before them to give you water, and you shall draw water from the rock and give drink to all the congregation.
In this parashah, the Torah tells us about the transgression of Moses and Aaron, spiritual leader and High Priest, which caused that both of them could not enter the Land of Israel. Aaron, although he does not strike the rock, receives the same penalty, because he was present when G-d gave the command to Moses, and nods when Moses strikes it instead of speaking.
The people of Israel are thirsty, they cry out to Moshe and Aaron to provide them with water. G-d tells Moshe to go to the rock and speak to it so that water will come out.
But incomprehensibly, instead of speaking, Moshe strikes it. No water comes out the first time, but when Moshe strikes the rock a second time, water does come out.
The question is, why did Moses and Aaron disregard the divine ruling? Why did they change G-d's order? If G-d had told Moshe to speak to the rock, why did he strike it?
Rashi explains that with this sentence, what G-d wanted to transmit to the people was the teaching that they had to be obedient to the word of G-d, in the same way that a rock, by simply speaking to it, gives water, obeying the divine order, how much more should man obey the precepts of G-d. Since instead of speaking to the rock he strikes it, not sanctifying the name of G-d, they were deprived of leading the people to the Land of Israel.
Moses thought that if I spoke to the rock, which is inert matter, and it fulfills the Divine command, tomorrow when I command the people and it does not comply, it will turn out that the people of Israel is harder than the rock, since the stone complies and the people does not. Therefore, there will be a very strong accusation against the people, that is why I prefer to strike than to speak.
Moshe's intention was good, since after all Moshe and Aaron as leaders of the people wanted to defend the people, so that there could not be a future accusation against the people in which it would be said that "the people are worse than the rock".
From this episode we learn that what is important is the action and not the intention. Being that Moshe and Aaron defaulted "...because you did not sanctify my Name by fulfilling the command that I had asked of you..." the consequence was "...you will not be able to bring the people into the Land of Israel..." From this moment on, it will be a new leader who will conquer the Land of Israel, Yehoshua Bin Num, Moshe's faithful pupil, from the tribe of Ephraim.
Why was the consequence so serious if the intention was good? The prophet is the channel of G-d's word in this world, and therefore, he has to abide by the divine ruling and order. The prophet cannot vary or change anything, he simply has to limit himself to fulfill what G-d speaks or reveals to him. If G-d tells Moses, in the presence of Aaron, to speak to the rock and instead he strikes it and nothing happens, the people could legitimately conclude that "...in the same way that Moses received the order to speak to the rock, and he did not fulfill it and nothing happened, he could also change other precepts...".
To teach us how serious it is to be able to alter the divine message, that is why there had to be such a serious consequence. And all this is so in order to ratify the rest of the laws, so that no one can one day conclude that Moses could change the laws.
From here we conclude that if in all the rest of the revelation there were no consequences, it is because there were no changes. This means that in all the rest they were faithful channels to the Divine message.