Quantum dot cellular automata (QCA), a nanoscale technology, are more prone to faults due to their diminutive size and precise fabrication process. The defects of QCA circuits have been the topic of many research papers, and this work has conducted a comprehensive study of the defect characterisation of QCA circuits. In this work, the defects in QCA circuits are divided into four groups: faults in the synthesis phase, faults in the deposition phase, faults in the design phase, and other faults. In addition, some unusual defects, such as stuck-at faults and single electron faults, are looked at. The study of the existing QCA works shows that fault-tolerant wires, tiles, etc. are rarely discussed in the literature. The literature also lacks adequate explanations of the QCA fault-tolerant clocking mechanism and fabrication processes. This work describes the identified challenges as well as their possible solutions. Additionally, this article outlines the future direction of this research and any relevant constraints. As a result, the intended audience can have a holistic understanding of the faults associated with QCA. This effort also seeks to facilitate the understanding of QCA circuit defects by related researchers.