Rather than an error, let us call the error-prone human faculties for decision-making imperfect, incomplete and containing very large areas of unknowability and unpredictable. Why is this? Because unlike a rock, which is quite "perfect" and "complete" and "predictable", the human is a complex and confusing set of innumerable and subtle relations of sensations. Some of these are outwardly oriented, such as in perception, some are inwardly oriented, such as in memory and imagination. Every human experience is a complex conglomeration of innumerable of these sense relations, and together they form a system of relatability-sensationality which is so vast and so deep that the "I", the unified-emergent function that is the "awareness" of this system (its ability to look upon itself, sense - interact with - itself as a "totality" and thus provide macroscopic direction to lesser more microscopic functions) cannot hope ever to grasp more than a small part of this large sensational realm. So most of what constitutes and gives rise to human consciousness-experience is unknown to us, which is to say unknown to the "sum" of this consciousness/experience itself, its emergent "I".
Thus every moment of consciousness emerges largely from what it itself cannot understand/comprehend in terms of its proprioceptive-regulating "I of awareness", its most unified emergent directionality. Consciousness' own causative and generative inner-relations largely fail to inform its own comprehension of its behaviors and outcomes, with certain relations ending up exerting an undo influence over the total entity with respect to its final actions (the activity of higher cognition as well as that of outward behaviors, i.e. what we are thinking, feeling or conceiving in the imagination at any given moment). This of course touches directly upon our ability to plan, think about our environment and project an inner representation of environmental conditions of the present as well as infer implied conditions of a possible future moment. Because all of this stems from a system of consciousness that is so largely unable to even sense/be aware of its own generating and sustaining "causes", a very large unpredictability and imperfection is introduced into the system. No two humans will react in the same way to a situation because that from which human reaction/behavior emerges is largely unique to each individual, and even more than this is unable to be called adequately forth into the present moment of judgement and comprehension that would allow it to inform a moment of consciousness.
One reason we may not see "deterministic" causation within ourselves to the same degree as we see it elsewhere (e.g. the falling rock) is because we are largely dependent upon and "caused by" what we cannot know or understand, what we cannot even see or directly sense. Yet because we nonetheless feel the effects and experience the outcomes of this "what we cannot see or sense" at every moment we are constantly frustrated in our attempts to understand the "reasons why we do things". Our judgements, decisions and actions (including our thoughts and inner feelings) are largely a mystery to us, we are left with the barest fleeting glance of what seems to be a large and largely invisible internal clockwork operating to produce for us a present moment of consciousness. This fleeting-superficial sense of perhaps one or several simple ideas, feelings, memories or sensations that we generally assume constitute the "reason why we did what we did".
The essential invisibility of this system to itself is an effect of the fact that this system is so massive and internally complex as to be largely unable to render itself unto itself, which is to say largely unable to act with a discernible unity and direction. The sufficient cause/s of our actions (again including our thoughts and feelings), if ever there were such causes, are always and already out of reach.
This imperfection, unpredictability can be seen as an "error" with respect to our self-cognition and comprehension of ourselves, which is also to say that it serves to explain why we err so often in thought, word and deed, but it seems perhaps more accurate to affirm that this complex-chaotic system of human consciousness must have been selected evolutionarily. It may be an "error" or lead to errors, but it is still a highly useful system. How might this be the case? One possible answer I can see is that perhaps the unpredictability, unknowability and even unreliability of consciousness has proven itself able to actually facilitate the survival of this animal species whose individuals (us) possess it. Maybe this sort of conscious set-up gives new expansive possibilities to the individual possessing it precisely because it is unclosed, incomplete and imperfect, and this heightened capacity for "random" or chaotic inner vision and sensational mediation served to strongly facilitate our survival.
(Topic adapted from a post at ILP forum)