The increasing demands for product variety have directed manufacturing companies towards accommodating flexibility by establishing mixed-product and mixed-model assembly lines. However, since greater variety leads to increased complexity, establishing these assembly lines becomes complicated. By conducting a case study, this paper investigates the causes of complexity and the applicability of assembly instructions in one mixed-product and four mixed-model assembly lines in a heavy vehicle manufacturing company. The results indicate a set of causes for complexity and highlight the significance of assembly instructions, as the practical implications for development of flexible assembly systems and design of products closely aligned with them.