proof rpoof porof oprof ropof orpof proof rpoof porof oprof ropof orpof poorf oporf poorf oporf ooprf ooprf roopf oropf roopf oropf oorpf oorpf profo rpofo porfo oprfo ropfo orpfo prfoo rpfoo pfroo fproo rfpoo frpoo pofro opfro pforo fporo ofpro fopro rofpo orfpo rfopo fropo ofrpo forpo profo rpofo porfo oprfo ropfo orpfo prfoo rpfoo pfroo fproo rfpoo frpoo pofro opfro pforo fporo ofpro fopro rofpo orfpo rfopo fropo ofrpo forpo poofr opofr poofr opofr oopfr oopfr pofor opfor pfoor fpoor ofpor fopor pofor opfor pfoor fpoor ofpor fopor oofpr oofpr ofopr foopr ofopr foopr roofp orofp roofp orofp oorfp oorfp rofop orfop rfoop froop ofrop forop rofop orfop rfoop froop ofrop forop oofrp oofrp oforp foorp oforp foorp
Note: these 'words' (valid or invalid) are all the permutations of the word proof. These words are obtained by scrambling the letters in proof.