The recipe for perpetual ignorance is: Be satisfied with your opinions and content with your knowledge.

Comfort is what stops learning, not stupidity. Once your opinions feel settled and your knowledge feels sufficient, there is no pressure to check either, and the gap between what you believe and what is true widens quietly for years. Nothing announces this; satisfaction feels exactly like understanding from the inside.

A small habit prevents most of it. Every so often, take a position you hold firmly and go looking for the strongest case against it, made by somebody who actually holds that view. Aim to state their argument well enough that they would recognise it.