srubk rsubk surbk usrbk rusbk ursbk srbuk rsbuk sbruk bsruk rbsuk brsuk subrk usbrk sburk bsurk ubsrk busrk rubsk urbsk rbusk brusk ubrsk bursk srukb rsukb surkb usrkb ruskb urskb srkub rskub skrub ksrub rksub krsub sukrb uskrb skurb ksurb uksrb kusrb ruksb urksb rkusb krusb ukrsb kursb srbku rsbku sbrku bsrku rbsku brsku srkbu rskbu skrbu ksrbu rksbu krsbu sbkru bskru skbru ksbru bksru kbsru rbksu brksu rkbsu krbsu bkrsu kbrsu subkr usbkr sbukr bsukr ubskr buskr sukbr uskbr skubr ksubr uksbr kusbr sbkur bskur skbur ksbur bksur kbsur ubksr buksr ukbsr kubsr bkusr kbusr rubks urbks rbuks bruks ubrks burks rukbs urkbs rkubs krubs ukrbs kurbs rbkus brkus rkbus krbus bkrus kbrus ubkrs bukrs ukbrs kubrs bkurs kburs
Note: these 'words' (valid or invalid) are all the permutations of the word srubk. These words are obtained by scrambling the letters in srubk.