Thomas Sowell points to central planning. I would add that the 'progressive' conviction that people are basically good along with the concomitant conviction that there is no such thing as radical evil is also deeply delusional, and also dangerously delusional.
Sowell also has wise things to say about 'under-representation' and 'over-representation.'
Do libertarians have a central delusion? I should think so. It is the tendency wildly to exaggerate the number of people who know their own long-term best interest. To properly qualify and explain this claim requires a separate entry.