Numbers 16:20-22 "And the Lord said to Moses and Aaron, 'Separate yourselves from among the congregation, that I may consume them in a moment.' And they fell on their faces, and said, 'O God, the God of the spirits of all flesh, shall one man sin, and wilt thou be angry with all the congregation?'" 

There's a bit of a rebellion going on among the Levites against Moses and Aaron's positions over the Israelites. Those contending for power have gathered at the tent of meeting, where Moses has said the Lord will make known who He has chosen to lead. Two-hundred and fifty men have gathered there, and the entire congregation has come as well, stirred up against Moses and Aaron by the chief antagonist, Korah. When God appears, He tells Moses and Aaron to move away from the rest so that He can destroy the entire congregation, but Moses and Aaron plead with God not to punish everyone for the wrongdoing of one man. (Numbers 16:1-24) 

This is an example, of which there are many, of Moses acting as a mediator with God. In the Old Testament, God frequently gets angry with his newly freed people as they keep showing their inability to trust and their tendency towards sin. The God we think of as "the Old Testament God" then threatens to take out His wrath on His people, but Moses steps in on behalf of their "stiff necks" and gets God to relent, at least somewhat. (Exodus 32:7-14)

These interactions are a foretaste of how Jesus Christ is our mediator with the Father. Moses was, among other things, a "type" for Jesus. Jesus is frequently portrayed by the Gospel writers as the "new" Moses. He is the perfection of what Moses was. Moses did not make it to the Promised Land because he doubted God at one point, but Jesus, who was obedient and trusting to the end, gave everything as the Sacrificial Lamb to obtain the Promised Land for all of us. And now that He has done this, Jesus is always interceding for us with the Father (1Timothy 2:5-7, Hebrews 7:24-25). As the Father's just wrath is kindled against us, we who still do not properly trust nor obey, and often rise up in challenge against God, Jesus shows His wounds to the Father and begs Him to relent.

At Moses and Aaron's intercession, God did relent in punishing the entire Israelite congregation, but He also did kill all two-hundred and fifty of the Levite challengers, as well as the entire households of two others who refused to come to Moses and Aaron's assistance. (Numbers 16:24-35) God is perfectly just, so He will have justice, but another aspect of God, an aspect we did not fully have revealed to us until the Son came, is His mercy. That is why the Old Testament God can seem so very different from the New Testament one. Although He never changes, He reveals different aspects of Himself to us in "the fullness of time," yet the Son is always at His right-hand, interceding for us and obtaining His mercy.