Avoiding dialogue breakdown is important in HRI and HAI. In this paper, we investigated why dialogue breakdown occurs. We hypothesize that confusion between elements and category is one important reason. Elements means individual strange utterances, and category means the entire ability and performance of a robot. We hypothesized that a user who confuses elements and category will tend to lose the motivation to continue interacting with a robot or agent when the robot or agent makes a strange utterance. To verify this hypothesis, we conducted an experiment. We asked participants to perform a memory task cited in a previous work. We separated them into two groups, the no-confusion group and confusion group, and we showed them a movie in which a robot made a mistake on a math problem. After that, we asked them in a questionnaire about their impression of the robot. We conducted a t-test between the two groups for each question. As a result, the participants who confused elements and category tended to brand the robot as having low ability and performance when it made a mistake, and those who were not confused did not have reduced trustworthiness in the robot or reduced motivation for continuing to interact when the robot made a mistake. These results supports our hypothesis.