PEM water electrolysis offers an efficient and flexible way to produce "green-hydrogen" from renewable (intermittent) energy sources. Most research papers published in the open literature on the subject are addressing performances issues and to date, very few information is available concerning the mechanisms of performance degradation and the associated consequences. Results reported in this communication have been used to analyze the failure mechanisms of PEM water electrolysis cells which can ultimately lead to the destruction of the electrolyzer. A two-step process involving firstly the local perforation of the solid polymer electrolyte followed secondly by the catalytic recombination of hydrogen and oxygen stored in the electrolysis compartments has been evidenced. The conditions leading to the onset of such mechanism are discussed and some preventive measures are proposed to avoid accidents. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.