exfl xefl efxl fexl xfel fxel exlf xelf elxf lexf xlef lxef eflx felx elfx lefx flex lfex xfle fxle xlfe lxfe flxe lfxe
Note: these 'words' (valid or invalid) are all the permutations of the word exfl. These words are obtained by scrambling the letters in exfl.