Members of the Heaven's Gate cult, who "knew" there was a spaceship following the Hale-Bopp comet, returned a perfectly good telescope they had purchased because they failed to see the spaceship they "knew" was there. Such behaviors demonstrate that

people will often go to extreme lengths to justify their actions or beliefs.

Explanation:

Often times, a belief is strongly associated with a certain set of life principle from which we develop our identity within the social group.

Abandoning this belief means that we have to admit that we have a false/ weak identity as a person. This will made people felt embarrassed or humiliated. This is why they often go to extreme lengths to justify their belief.

Other example would be a group of flatearthers.

Rather than believing the proven image given by known scientists, they choose to believe their fellow flat earthers who have no relevant education in the matter at all. They developed a belief that the society is trying to trick them because changing their beliefs would made them felt humiliated.