Missed medication doses can affect patient safety in hospitals. The use of automated dispensing cabinets (ADC) in clinical areas may reduce the occurrence of medications being unavailable at the time of need by alerting pharmacy to replenish stock. However, workarounds in ADC use can affect this capability. We studied nurses’ use of an ADC in a paediatric intensive care unit with  54 hours of observation, semi-structure interviews (19 nurses; 4 pharmacy/management staff) and review of reported incidents. We found running out of medication a recurrent problem despite the ADC. Contributing factors affecting data entry in the ADC, and therefore replenishment alerts to pharmacy, included: the organisation of medication activities in the ward, nurses’ information needs, patients’ medication needs, and ADC design. Running out of medications was a ‘tragedy of the commons’. Effective solutions may require going beyond user training and directives for accurate data entry, to work process redesign.