What scene or incident from Voltaire's Candide illustrates an aspect of Voltaire's understanding of human nature?

Voltaire had a pessimistic view of human nature, finding it largely irrational. This is illustrated in chapter 28 in a scene where Candide has a conversation with Pangloss. Pangloss recounts that he has survived a hanging and only managed not to be dissected by screaming on the autopsy table at the first incision. Then, he tells Candide, he entered a mosque and picked up the nosegay a woman there had dropped. He appeared to be looking at her cleavage as he returned it and so was sentenced to one hundred lashes on the soles of his feet. Then he was made a galley slave, and in his words:

We were continually whipped, and received twenty lashes a day with a heavy thong

In this particular scene, Pangloss recalls all these horrors. Candide asks him if his recent sufferings have caused him to reconsider his adherence to Leibniz's theory that all is for the best in the best of all possible worlds:

"Well, my dear Pangloss," said Candide to him, "when You were hanged, dissected, whipped, and tugging at the oar, did you continue to think that everything in this world happens for the best?"

Remarkably, Pangloss does adhere to this philosophy, saying:

it would not become me to retract my sentiments; especially as Leibnitz could not be in the wrong: and that preestablished harmony is the finest thing in the world

Clearly, this is highly irrational, and Voltaire is pointing out how senseless human beings are to value a theory above the reality of lived experience. Today, we call this confirmation bias, and unfortunately, this tendency to "know what we know" in the face of overwhelming evidence to the contrary is still prevalent.