In last week's Torah portion, Shelach, we read about the Chet HaMeraglim, the Sin of the Spies, an incident that occurred less than a year after the Chet HaEgel, the Sin of the Golden Calf. These events are often pointed to as the two greatest tragedies in Jewish history because of what they presaged for future generations.
In the story of the Golden Calf, Moses beseeches God with His "13 attributes of mercy" (Exodus 34:6-7). In the story of the Spies, Moses invokes an abbreviated version of these attributes (Numbers 14:18). Let's briefly look at how God manifests his merciful attributes in these two instances.
In the incident of the Golden Calf, God threatens to destroy the Jewish nation but Moses convinces Him otherwise and God then "repented of the evil which He said He would do unto His people" [a phrase that always raises considerable theological difficulties]. Afterward, Moses tells the Levites that God instructed them to slay the offenders and they proceed to kill 3000 of their brethren. Moses again asks God for forgiveness, and God responds that He will mete out punishment to those who have sinned. An additional unspecified number of people are then killed by God. One would think that all of this death and destruction would be sufficient atonement, but no - it is a commonly accepted belief that every tragedy that befalls the Jewish people to this day has within it some retribution for the sin of the Golden Calf (see, for example, Rashi on Exodus 32:34.)
In the incident of the Spies, God again threatens to entirely destroy the nascent Jewish nation, but instead responds to Moses' pleas by sentencing everyone over the age of 20 - except for Joshua and Caleb - to die during the sojourn in the desert (various midrashic accounts exempt the women and/or the Levites.) The 10 leaders are killed in a plague. There is a particularly horrifying midrash about people digging their own graves every Tisha B'Av and being made to sleep in them, not knowing whether they will survive until morning. Furthermore, Rabbi Akiva (Sanhedrin 107b) states that the entire generation of the wilderness has no share in the world to come!
Both stories show a people experiencing a primeval fear and then responding to this fear in a very human way. According to many commentators, the Golden Calf was nothing more than a surrogate leader created out of desperation when they feared the loss of their leader Moses. The despair of Bnai Yisrael in the Spy story resulted from panic at the thought of certain death for themselves and captivity for their wives and children.
The typical response within Orthodox circles is a glib one: "They experienced miracles! They experienced God's direct revelation! They should have trusted in God, they should have trusted Moses." Yet tradition unequivocally claims that this was a nation that had known only slavery for generations and had also been quite assimilated into Egyptian culture. To undergo a radical - and more importantly, permanent - transformation takes considerable time and effort, but God seems to demand instantaneous results from a people who had internalized an Egyptian slave mentality over hundreds of years and as a result had reached the lowest level of impurity possible.
I would argue that both stories do not describe a God who displays attributes of mercy but instead One who reacts in a manner that suggests that He doesn't understand human psychology very well and/or is unreasonable in his expectations of human behavior.