Motivation – Our study is designed to identify human factors that are a threat to the safety of children with heart disease. Research approach – After an initial observation period, we will apply a major safety intervention. We will then re-measure the occurrence and types of human factors in the operating room, and the incidence of adverse events, near misses and hospital death, to evaluate if there was a significant post-intervention reduction. Findings/design – We focus on challenges encountered during the training of the observers. Research Limitations – Because of the complexity of the OR, observations are necessarily subjective. Originality/Value – This work is original because of the systematic evaluation of a safety intevention and the training protocol for the observers. Take Away Message – Systematic and periodic assessment of observers is required when teamwork is observed in complex, dynamic settings.