Moses immediately does the right thing. He takes his issue up with the Lord to consult his help. This is something we should all do with any trials or difficult positions in which we find ourselves.
God tells Moses to speak to the rock, and water will come out. However, Moses decides God's way is no longer the best way to solve the problem and decides instead to strike the rock as he had done before in Exodus 17.
This disobedience to the Lord would ultimately cause Moses not to be able to enter the Promised Land. As Numbers 20:11-12 states,
"And Moses lifted up his hand, and with his rod he smote the rock twice: and the water came out abundantly, and the congregation drank, and their beasts also. And the Lord spake unto Moses and Aaron, Because ye believed me not, to sanctify me in the eyes of the children of Israel, therefore ye shall not bring this congregation into the land which I have given them."
Clearly, Moses made a mistake, and this mistake kept him out of the Promised Land. Simply, it was his fault and his alone that he was unable to enter therein.
Yet, in the book of Deuteronomy, commonly considered Moses' farewell address, Moses mentions nothing of this. In Deuteronomy 1:37, Moses proclaims.
"Also the Lord was angry with me for your sakes, saying, Thou also shalt not go in thither."
"For your sakes." On two other occasions (Deuteronomy 3:25-27 and Deuteronomy 4:21), Moses blames the Israelites for his own actions, saying the Lord would not let me into the Promised Land "for your sakes." In this area, Moses refuses to take responsibility for his own actions, instead blaming it on the Israelites. Although the point can be made that the Israelites caused his sin by putting him in a precarious situation, it was still his responsibility to handle that situation with obedience to the Lord. In the end, he needed to take responsibility for his own actions and not blame it on someone else.
Of course, we need to do the same. Even the most humble servant of the Lord is not immune to making mistakes. But what is perhaps most telling of our character is how we respond. Do we take the blame or do we find someone else we can assess as the cause of all our problems?