Per-oral endoscopic myotomy (POEM) is a relatively novel endoscopic technique for the treatment of achalasia. POEM has been shown to have outcomes comparable to those with Heller myotomy, but it is less invasive and has fewer complications. A 72-year-old man with progressive solid and liquid dysphagia underwent POEM, but soon after the procedure went into cardiac arrest; spontaneous circulation returned after 10 minutes of CPR. He was subsequently found to have tension pneumopericardium as a result of the inadvertent use of air instead of carbon dioxide during the procedure. He had a prolonged hospitalization that required an extended stay in the medical intensive care unit. Although rare, POEM can lead to critical, life-threatening complications.