Occipital nerve block is usually considered to be a very simple and safe regional anaesthetic technique. We describe a case of sudden unconsciousness during a lesser occipital nerve block in a patient with an occipital bone defect. A 63-year-old man complained of headache, which was localized to the right occipital region. A right lesser occipital nerve block with a local anaesthetic was performed for treatment. During the lesser occipital nerve block, the patient suddenly became disturbed and lost consciousness. Two hours after the incident, the patient was fully awake without neurological sequelae. He had previously undergone a microvascular decompression for right trigeminal neuralgia. The patient had a bone defect following craniotomy. We believed that the loss of consciousness during lesser nerve block may be due to a subarachnoid injection. Occipital nerve block is relatively contraindicated in the presence of a bone defect.