I started to add this little tale to Nish's post about self-inflicted system zapping, but it is too long and would hijack a fun Sandbox thread, so here it is.
I just updated an old test box of mine. Afterwards, I found that the keyboard had "somehow" gotten changed from US English to Turkmen, so all the characters I typed looked very weird to me. Of course, when looking at the PCC>Hardware>Keyboard list of choices, it looks easy to pick the wrong one - Turkmen is just above US English on the list, but I still don't know how I did that. Anyway, I just happily changed my keyboard back to English and went on my merry way.
Then I rebooted. Neither root nor user password would let me in - gaaakkk!
I rebooted again into safe mode, brought up mcc and changed the keyboard back to Turkmen. Now I could boot back into KDE, so I opened PCC and got the keyboard setting ready to be changed. Then, before I made the change back to English, I opened a terminal and logged into su with the Turkmen password. Next, I went to PCC and changed the keyboard back to English. Issuing the passwd command, I was then able to change the root password back to something without them furrin letters.
There may be an easier way (short of not frakking it all up in the first place!), but at least this worked.