Running Windows XP I had upgraded from 3.5 to 3.6. After doing so my spell checker would not work. It would run through the document and find nothing. Auto spell-check wasn't picking up anything either.
Platform (if different from the browser): w.XP
By deleting all the user data for LibreOffice and then restarting LibreOffice spell check began to work again.
*** This bug has been marked as a duplicate of bug 53006 ***