Emergency department (ED) attendances in the UK have increased significantly during the past 5 years, resulting in 23.4 million attendances in 2016/2017.1 Crowding is now a familiar challenge in UK EDs. Crowding describes overwhelmed EDs unable to operate effectively; one consequence is that suboptimal care is delivered to patients, because longer waiting times delay diagnosis and treatment.2 This can lead to significant negative outcomes including a reduced quality of care and increases in length of stay, serious incidents and mortality.3–6 Workforce challenges, including difficulty in recruiting and retaining the nursing and medical workforce, also occur.7 8 Crowding contributes to clinically significant delays in diagnosis, recognition of acute deterioration and the commencement of treatment; patients with time-critical conditions are particularly vulnerable. The adult ED at Bristol Royal Infirmary is situated in an inner city centre setting, with an annual attendance of over 70 000 new patients. If there is no immediate capacity for patients arriving by ambulance, then after an initial assessment, the patient is moved to the corridor outside the ED on a trolley and ongoing care is provided by a ‘queue nurse’, who may be unfamiliar with the ED environment and associated clinical quality standards. We found in retrospective reviews of clinical incidents reported during times of crowding that variation in practice and omissions in basic elements of care were common contributory factors to incidents. Human factors also play a central role in the delivery of substandard …