Early warning scoring is designed to be an objective tool to aid identification of hospital patients at risk of deterioration. 'Track and trigger' systems using such scores are widely used but many aspects of scoring have not been clarified. We aimed to document how observations and scores are used in practice as part of a typical track and trigger system. We extracted patient observations and early warning scores from the casenotes of 189 patients admitted to Furness General Hospital during a large outbreak of Legionnaires' disease in 2002. We used these 3739 sets of primary observations to recalculate scores, and compared them with those recorded in the casenotes. Recording of patient observations was variable. Early warning scores were derived from 2607 sets of observations (69.7%), of which 571 (21.9%) had been incorrectly calculated. Incorrect scoring meant that 66 of 270 patients (24.4%) whose observations should have reached the trigger value did not. Patients with more abnormal observations were more likely to be misscored. Scoring errors were more likely to lead to underscoring as the degree of physiological abnormality increased. Patients with confirmed Legionnaires' disease were more likely to be incorrectly scored. We conclude that the assignment of early warning scores is prone to error and this may delay referral of at-risk patients for critical care management.