An ultrasound-guided thoracic paravertebral nerve block was administered for left-sided abdominal pain. Lidocaine 0.4% (9.5 mL) was injected. Spinal anesthesia developed after the injection that resolved after 3 hours. One week later, a magnetic resonance imaging scan of thoracic spine identified a left T10 nerve root sheath cyst. It was postulated that the local anesthetic was injected into the cyst, which communicated with the subarachnoid space. This case report demonstrates the anatomic variations of nerve root sheath cyst sites and the risk of injection into intranerve root sheath cysts during interventional procedures.