Lumbar discography is a diagnostic modality to determine whether the intervertebral disc is the cause of pain. The injection of radiopaque contrast into the nucleus pulposus of the disc can reveal the internal details of the disc. We describe a case of inadvertent lumbar discogram resulting from an attempted lumbar interlaminar epidural injection at L₅–S₁ under fluoroscopy. The patient did not have a postdural puncture headache or nerve root irritation. The potential triangle in the lateral aspect of spinal cord may be the explanation for this situation, because this triangle is composed of the exiting nerve root laterally, the lateral margin of the dura medially, and the pedicle as its base. The L₅–S₁ disc is located in the center of the triangle. In our case, the Tuohy needle was placed possibly in the center of the triangle, too laterally to puncture the dura. Hence, the patient did not suffer from postdural puncture headache. The needle was probably inferior to the nerve root, and no obvious nerve root trauma or irritation occurred. This potential triangle may provide alternative access for lumbar discography at the L₅–S₁ level.