This article discusses the use and possible misuse of Maximum Experimental Safe Gap (MESG) in selecting deflagration and detonation flame arresters (DDAs). Gas mixtures having low autoignition temperatures or unusually wide flammable ranges might not behave as predicted on the basis of MESG, especially under restricted-end deflagration conditions. When applied to DDA selection the Westerberg apparatus might prove to give a more reliable ranking of gas sensitivity than the IEC apparatus. If credit is taken for inert gases using the new MESG combination formula in NFPA 497 an incorrect DDA could be selected. An outline is given for a possible test program to elucidate the effect of gas mixture MESG on DDA performance.