Various mechanical problems with the delivery of anesthesia gases have been reported, and preoperative checks of the anesthesia delivery system are designed to prevent these. We report a case of a mechanical obstruction of the expiratory limb of the anesthesia circuit which went undetected before the induction of anesthesia. A check of the circuit was performed before the addition of circuit extensions, thus missing the obstruction. Systematic investigation of the anesthetic system after difficulty in ventilating the patient revealed the obstruction, without any harmful consequence to the patient. This case highlights the importance of a thorough check of the system before any anesthetic, particularly after any change in the circuit.