The LORD said to Moses, “Take the staff, and you and your brother Aaron gather the assembly together. Speak to that rock before their eyes and it will pour out its water. You will bring water out of the rock for the community so they and their livestock can drink.” So Moses took the staff from the LORD’s presence, just as he commanded him. He and Aaron gathered the assembly together in front of the rock and Moses said to them, “Listen, you rebels, must we bring you water out of this rock?” Then Moses raised his arm and struck the rock twice with his staff. Water gushed out, and the community and their livestock drank.
But the LORD said to Moses and Aaron, “Because you did not trust in me enough to honor me as holy in the sight of the Israelites, you will not bring this community into the land I give them.” Numbers 20:7-12

What did God ask Moses to do? Speak to the rock before the people.

What did Moses do? Scream in anger at the people then hit the rock twice.

Why is this a big deal?

Moses showed frustration toward the people. This is God’s job. Moses lost something here – heart, humility, empathy. It was about him for this moment. He embarrassed himself and lost some respect.

He hit the rock – not once – but twice. The first angry strike could have been forgivable. Twice, indicates rage. Angry at God? Maybe.

God caused plagues to wipe out thousands when they were disobedient. He needed to judge Moses in some way for his disobedience. Leaders don’t get a free pass. They live as examples. As a result, Moses could not see the result of all his labor. Aaron too was sentenced to die, since he appears he had some part in the whole thing.

Moses started to adopt the rebellion of his people. This became a dangerous precedent.  God had to deal with it.