To identify the actual cause of the overpassivization errors made by L2 learners, we examined three main previous studies. The first approach is to lay the responsibility to the existence or absence of the information about the external cause (Ju 2000; Chung 2010). The second approach is to ascribe the reason to the transitive counterpart of the unaccusative verb (Yip 1995; Chung 2010). The final approach is to consider the adversity effect as a reason for the overpassivization error (Jung 1996a, 1996b). By conducting an experiment, we show that each factor should not be considered in isolation. Instead, we suggest that all these factors are significantly relevant in explaining the overpassivization errors made by Korean learners.