There are many consequences and harms of conceit, and their list is as follows:
2. forgetting a sin and underestimating it
3. deprivation of the benefits of admonishment and guidance
4. indifference to people's ways of harm
5. non-belief in the mercy and favor of Allāh
Any of these consequences suffices to render one's perdition and suffering; so, what would you say about all of them combined?!