Books can have many tags. Please examine the list of tags for the book you believe should be visible in the VL. I suspect you will find that it after all has the tag Apples or Oranges.
Otherwise you are right that something very strange is going on. You may have had your copy of Calibre changed by high energy cosmic radiation or as an act of some mischievous godlike creature. Or something even stranger. You should be able to rule that out by updating to the latest version of Calibre.
If that doesn't help, the strange error may be in the database. Run "Check Library" to see if that helps. If not, make a copy of the book. Delete it from Calibre. Add the book back to Calibre, and see if things work as they should.
If the problem still persists, there could perhaps be strange metadata for the book. Please post a copy of the OPF-file for the book that misbehaves here. Or edit the OPF-file and paste the contents here. That could perhaps make it possible for others to reproduce the problem and figure it out. (And verify that there REALLY are no "Apples" or "Oranges" in the tags.)
DaltonST: Your description does not seem to match how it works when I try to reproduce the problem. Or perhaps I have gremlings in my computer as well...
