Objective The HEART score can accurately stratify the risk of major adverse cardiac events (MACE) in patients with chest pain. We investigated the frequency, circumstances and potential consequences of errors in its calculation. Methods We performed a secondary analysis of a stepped wedge trial of patients with chest pain presenting to nine Dutch emergency departments. We recalculated HEART scores for all patients by re-evaluating the elements age (A), risk factors (R) and troponin (T) and compared these new scores with those given by physicians in daily practice. We investigated which circumstances increased the probability of incorrect scoring and explored the potential consequences. Results The HEART score was incorrectly scored in 266 out of 1752 patients (15.2%; 95% CI 13.5% to 16.9%). Most errors occurred in the R (‘Risk factors’) element (61%). Time of admission, and patient’s age or gender did not contribute to errors, but more errors were made in patients with higher scores. In 102 patients (5.8%, 95% CI 4.7% to 6.9%) the incorrect HEART score resulted in incorrect risk categorisation (too low or too high). Patients with an incorrectly calculated HEART score had a higher risk of MACE (OR 1.85; 95% CI 1.37 to 2.50), which was largely related to more errors being made in patients with higher HEART scores. Conclusions Our results show that the HEART score was incorrectly calculated in 15% of patients, leading to inappropriate risk categorisation in 5.8% which may have led to suboptimal clinical decision-making and management. Actions should be taken to improve the score’s use in daily practice.