tdnf dtnf tndf ntdf dntf ndtf tdfn dtfn tfdn ftdn dftn fdtn tnfd ntfd tfnd ftnd nftd fntd dnft ndft dfnt fdnt nfdt fndt
Note: these 'words' (valid or invalid) are all the permutations of the word tdnf. These words are obtained by scrambling the letters in tdnf.