Circular OCT around the optic disc producing measurements of the peripapillary retinal nerve fiber layer. The OCT scan of the left eye is normal and the algorithm for identifying the anterior and posterior borders of the retinal nerve fiber layer has functioned correctly. In contrast, the scan of the right eye has failed to identify the posterior border (blue arrow) and the anterior border (red arrow). When the thickness graph below the image of the right eye goes to zero or goes very, very above the much above the end of the Y‑axis, the clinician should suspect algorithm error.