One aspect is that Communist theory falsely assumes is human vices like avarice and greed can be evolved out of the human species after only a generation or two through forced labor and deprivation; but that's not how evolution works.
A second aspect is "From each according to his ability, to each according to his needs". Unfortunately, those who decide what is best for everyone also seem to be the most "needy" -- it is inevitable that those in charge will take more than their fair share of the collective wealth and keep it for themselves.
A third aspect is that, due to the previously-mentioned aspects, every Communist nation eventually devolves into feudalism, with a dictator/king at the top, serfs/workers at the bottom, with soldiers/knights keeping order everywhere.
That's basically it: In theory, there is no difference between theory and reality; but in reality, there always is.
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The mere fact that science may not yet adequately explain an object, event, or experience does not mean the immediate explanation should automatically default to a conspiratorial, extraterrestrial, paranormal, or supernatural cause.