Numbers 20:10-13. The people have been complaining about not having water in the desert, so God has told Moses and Aaron to bring the people before the rock at Meribah and tell it to bring forth its water so that the people and their cattle can drink. Moses and Aaron bring the Israelites to the rock and say "[S]hall we bring forth water for you out of this rock?" Moses then strikes the rock twice with his rod, and the water comes forth abundantly. But the Lord tells Moses that because he struck the rock he will not be allowed to lead the people into the Promised Land, for he did not believe in the Lord and did not sanctify the Lord in the sight of the people.

It seems such a minor transgression, striking the rock, for such a strong punishment, not being allowed to enter the Promised Land. But God told Moses exactly what to do, and Moses was accustomed to talking and listening to God, he knew what God was saying to him. If God wanted Moses to strike the rock, he would have told him to do that. He didn't. He told Moses to simply tell the rock to send forth its water. That would have been so much more obviously a miracle and would have really shown God's love and power. Instead, Moses and Aaron steal a bit of God's glory by saying "Shall we bring forth water?" And then doing something which could have caused the water's release.

Details are important to God - he is accomplishing something with everything that happens. When he chooses someone, like Moses, to become close to him and really talk to him, he expects that person to trust and believe in him. It is through following God's will as precisely as we can that we can help him accomplish all he intends to.