Sinking skin flap syndrome (SSFS) or “syndrome of the trephined” is a rare complication that can occur after decompressive craniectomy. Disabling neurologic deficits, as well as the impairment of overall mental status with the development of a concave deformity and relaxation of the skin flap, are frequently observed. This usually develops several weeks to months after craniectomy. The pathophysiology of the syndrome includes cerebrospinal fluid (CSF) hypovolemia and the development of an atmospheric pressure gradient that can be worsened by CSF diversion, dehydration, and change in position such as can be seen with a lumbar puncture. We present a case of a 40-four-year-old male with calvarial multiple myeloma three months after craniectomy who developed SSFS two days after lumbar puncture was performed to investigate possible leptomeningeal spread. It is imperative to recognize the syndrome early and proceed with urgent management with measures that initially increase intracranial pressure such as IV hydration and Trendelenburg positioning. In certain cases, proceeding with surgical management, such as epidural patch or cranioplasty, can be life-saving.