Human error and personal failure are inherent to the human condition. The question is how the costs shall be defrayed. In the case for liberty, there is certainly some tolerance for failure or error, as it is generally suffered by the individual, not brought upon anyone by design. However, wherever anyone seeks to empower government in an effort to meet these ends, one must be absolutely certain of its effects, as there is no margin for error behind the barrel of a gun.
One must remember that government is a monopoly on force and coercion. It is force and coercion not by spirit or intention, but in accordance with the letter, understanding and integrity of the enforcers in their own time. As opposed to a state of liberty, where mistakes, failures and crimes are unavoidable and still likely to happen, a state of socialism condemns the people to a state of misery, failure and crime by design.
A system so brutal, so uncompromising and violent simply cannot afford any measure of uncertainty. For this reason, the Leftist must, for the sake of humanity, satisfy the highest standard of certainty, so as to erase every doubt as to any potential failures. The burden of proof is everywhere, and on every occasion, on the Leftist or, more broadly, any who seeks to undermine or otherwise threaten or redefine the terms for life, liberty, or property.
Of course, if the Leftist were to theoretically satisfy this standard, and if he were to, by some miracle, have a moment of such honesty and sobriety — perhaps some epiphany through divine intervention or by the Grace of God — he would no longer be a Leftist, but rather a staunch advocate and defender of liberty, a classical liberal in the truest sense. Therefore, there is no Leftist who can satisfy that standard, and there is no rational case at all for Leftism; it is dangerous enough to entertain the thought of it, let alone to condemn the people to its abuses.
The case for Leftism is dubious enough on its own, but especially so where such an alternative as liberty is at our fingertips, specifically because it has been so articulated by our forbears, and the fruits so bountifully produced throughout history. We needn't commit the mistake of entertaining that which has caused so much grief and misery, which has brought so much pain and suffering, and which has claimed so many lives (countless tens of millions by body count alone), and that which ultimately cannot be made to work.