A 49 year-old male was scheduled for a cholecystectomy, thereafter a 37 year-old female scheduled for removal of a epidural hematoma in the same operating room. Both of them had no specific medical problems and past medical histories for anesthesia. For those reasons, anesthesia was induced with thiopental sodium and succinylcholine with endotracheal intubation. After induction, vital signs including body temperatures were stable. But moisture dew in the unidirectional valves and corrugated tubes, and color changes of soda lime were discovered. At that time, severe hypercarbia was recognized by arterial blood gas analysis in both cases. In both cases, there were no malfunctions in unidirectional valves, expiratory valves, corrugation tubes, soda lime, ventilators and there connection parts in the anesthetic machines. Also there were no abnormalities of blood pressures, electrocardiograms, pulse oxymeters, temperatures and the pulse in the patient monitoring systems except capnography. At first, we thought that medical signs revealed malignant hyperthermias. But vital signs, air way pressures and functions of all kinds of anesthetic machine components including ventilators were normal. After discontinuing N2O gas deliveries in the operation room, hypercarbias disappeared. Thus, anesthetic gas delivery systems via central piping systems were checked and it was discovered that CO2 gas was in the N2O gas tank instead of N2O.