## 1 Introduction  
An efficient and reliable assembly process is a critical aspect of manufacturing, ensuring that the final product meets the required quality level. Engineers usually consider several variables when selecting an appropriate assembly system. Among them are flexibility, productivity, product variants and production volume [1]. Figure 1 shows the relationship between some of these variables and the level of automation in the assembly system. The development of industrial robotics has induced a significant increase in the automation and productivity of manufacturing processes, including assembly [2]. However, in manufacturing domains where product complexity and variety present particular challenges, manual work remains a viable alternative. This is the case of such manufacturing domains as consumer electronics [3], aerospace manufacturing [4, 5], combustion engine assembly [6], automotive manufacturing [7, 8] and the production of industrial machines and tools [9, 10]. Hence, for manual assembly to yield a final product of the appropriate level of quality, several operations must be executed properly. For example: selecting, handling and fitting parts; checking; applying force; and retrieving and analyzing information. Unfortunately, these operations are susceptible to human error and represent potential sources of defects. [PERSON] and [PERSON] [11] define human errors as \"any member of a set of human actions that exceed some limit of acceptability, i.e., an out-of-tolerance action, where the limits of tolerable performance are defined by the system\". Common error-related defects in manual assembly processes include loose connections, missing components, the installation of wrong components, improper application of force to fasteners,damage during assembly and contamination by foreign object debris [12; 13]. Poor physical and cognitive ergonomics have been found to increase errors and impact product quality [12; 14]. The links between human factors, errors and quality issues have been recognized in the literature [15]. Human errors are to be avoided to limit economic losses associated with defects and unnecessary waste. For this reason, ergonomics and human factors discipline is considered to be a key practice area within the lean manufacturing approach [12]. With the introduction of Industry 4.0, manufacturing will experience an increase in product customization under the conditions of highly flexible (large scale) production [13]. According to [PERSON] et al. [14], this mass customization is expected to escalate production complexity and could increase the demands on human operators' skills. In this context, optimizing the working system is necessary to minimize human error. Identifying and understanding the factors that affect assemblers' performance is the first step in the implementation of effective strategies. [PERSON] [15] argues that human reliability is crucial to improve quality in the manufacturing sector. However, human reliability research has mostly focused on safety, while less attention has been paid to the manufacturing sector and to manual assembly specifically [16]. To close this gap, this paper applies classic human reliability analysis (HRA) techniques to a realistic case study of complex manual assembly. It assumes that some HRA techniques, developed and mostly used in safety-critical domains, can also be useful in the analysis of human error in manual assembly context. The study's main goal is to understand where and why manual assembly errors occur by identifying the most common error modes, evaluating them, and determining the factors that influence the assemblers' performances. The paper also discusses potential strategies for the reduction of errors, including new technological approaches. Figure 1: Utilization areas for manual, hybrid and automated assembly concepts. Reprinted by permission from Springer: Changeable and Reconfigurable Assembly Systems by [PERSON] and [PERSON] (COPYRIGHT 2009).