A critical incident is described where the expiratory (scavenging) port of an obsolete version of a Bain-type breathing system valve assembly was inadvertently connected to the common gas outlet of an anaesthetic machine. This resulted in the patient being deprived of a supply of fresh gas. This misconnection was made possible by adding a plastic connector to the valve scavenging port and subsequent wrongful misconnection. The patient fortunately suffered no harm. The case highlights the danger of equipment that has been subjected to unauthorized interference.