Revolutionaries thinking that only if they terrorize enough people a new better society will magically come into existence.
And of course they will be the new ruling class, never on the receiving end of the terror.
Anti-communists thinking that by doing blanket condemnations of past mistakes instead of historical and material analysis of why it happened, how much was necessary, and how much was the excess, they can totally avoid them in the future and bring down capitalism with the power of love.
How many times does the same mistake have to repeat? Communists didn’t invent revolutions you know. Peasant rebellions were a thing in medieval Europe, and many different kinds of uprisings were tried during the centuries. And there’s the same pattern repeating again and again - it either fails in bloodshed, or succeeds only for the winners to establish a new tyrannical system.
The only exception was started by rich landowners because they didn’t want to pay taxes to the king. (American)
Note that I’m talking about violent revolutions - there were quite a few examples of non-violent or semi-violent revolts/uprisings that didn’t end up catastrophically. India, South Africa, Portugal, post-communist Eastern Europe come to mind.