Despite significant diagnostic and technical progress in managing intracranial aneurysms, there are still open questions in understanding their pathophysiology: how fast can they form and grow? We had the chance to observe the "de novo" genesis and rupture of an aneurysm of a left MCA posterior trunk M3 branch within 14 days in one of our patients. We were in the position to compare an initially inconspicuous vessel, assessed during a diagnostic cerebral angiogram with 3D acquisitions, performed as an elective follow-up to monitor the decade stability of a transitional aneurysm in the same vascular territory, and the same vessel only two weeks after, harboring a new small ruptured aneurysm. Several studies along the intracranial aneurysms' pathophysiology have been reported but primarily oriented toward identifying uncommon conditions such as inherent defects in collagen synthesis, genetic or familial factors, or basic anatomic variations or abnormalities in the cerebral vasculature. Suppose this case report does not pretend to provide a clear answer to these questions. However, it is up to date, the shortest time (14 days) reported in the literature for a well-documented "de novo" genesis and rupture of an intracranial aneurysm "in vivo" in humans. The purpose of this case report is not only to underscore the unpredictability of this vascular disease but, even more, to support the idea that further investigation, with more modern methodologies, is of paramount importance in determining the etiopathogenesis and behavior of this stealthy disease.