This study focuses on the analysis of peripheral phenomena such as verbalization and over-learning by users while playing serious games (SGs) in order to develop their competences. For this purpose, we used an SG aimed at training the operators who are meant to work in cleanrooms devoted to pharmaceutical production. Data concerning the effectiveness of developing competences will be interpreted here, as well as the unexpected results concerning the way the SG is actually used in a naturalistic context. Indeed, some additional phenomena such as flow, immersion, thinking aloud, problem solving and open discussion will be presented in this paper.