The delivery of an hypoxic gas mixture to a patient during general anaesthesia is a rare event due to contemporary standards of monitoring, equipment design and alarm features. An incident is described where a split occurred in a rubber seal round the top of a flowmeter control tube. This resulted in a downstream oxygen leak and the delivery of an hypoxic gas mixture to the patient. The bobbin on the oxygen flowmeter did not accurately reflect the amount of oxygen being delivered. A paramagnetic oxygen analyser and a fuel cell oxygen electrode indicated that the inspired oxygen concentration was lower than intended. The anaesthetic machine was exchanged, and the operation continued uneventfully. The faulty anaesthetic machine subsequently passed a formal pressure test by the hospital engineers and also close examination of the flowmeter control valves. The importance of monitoring equipment and the interpretation of the information that they provide is emphasized.