The children of Israel committed two serious sins in the wilderness – the Sin of the Golden Calf and the Sin of the Spies. In both cases, God was about to wipe them all out and, in the merit of Moshe's prayers and entreaties, He decided to forgive them. God would send a plague to wipe out just part of them but not destroy the entire nation completely.
In Tractate Rosh Hashana 17b, our Sages explain that it was God Himself who taught Moshe exactly what to pray when He is angry with Israel. He gave Moshe the 13 Divine Attributes of Mercy and told him that when they are recited, they bring mercy, grace and forgiveness into the world, including to sinners.
In Numbers 14, Moshe follows God's instructions in order to save the children of Israel after the sin of the Spies, but he doesn't list all the 13 Attributes (verse 18). Why does he choose to omit some of them?
Moshe specifically chooses those Attributes he feels are most relevant, based on the relationship between God and Israel at this particular time.
Moshe omits the first of the two Divine names. Our Sages explain that God's name appears twice to acknowledge the connection between Him and a person both before and after the sin. Since Moshe is now speaking after Israel have already sinned, he cannot mention the first Divine name.
Moshe then omits the next Attributes, "God, Compassionate and Gracious," since they are a request for a gift. He is not asking for a gift, but rather forgiveness for the sin that had already occurred.
He omits the Attribute of "Truth" since this would likely bring Divine judgement upon Israel. Israel had tested God after all the miracles that they had witnessed. "Forgiver of …. Error" is also omitted because they had deliberately sinned against God. It was not accidental or due to lack of knowledge, rather a deliberate provocation against Him.
Moshe is teaching us that whenever we turn to God we must be aware of what we are saying and requesting and not to pray by rote.