Incidents caused by human errors such as ‘thoughtlessness’ and ‘carelessness’ are likely to occur in a general laboratory because the nature of work frequently involves manual evaluation and input. In this article, we report the incident analysis of the general laboratory of our hospital and discuss examples of the measures taken to prevent such incidents from occurring. Fourteen incidents that occurred between April 2009 and October 2016 (92 months) were analyzed in terms of their influence level, causes, time of incident occurrence, and preventive measures. The main causes of the incidents were mistakes (10 cases), lack of knowledge (2 cases), carelessness (1 case), and insufficient information transfer (1 case). The incidents frequently occurred on Mondays and Tuesdays, during lunch time (11:00–13:30), and during day and night duties. It is important to develop countermeasures to create an environment that minimizes the occurrence of mistakes and to develop simple and easy verification methods.