Schrodinger's wife to Schrodinger:
What the **** have you done to the cat? It looks half dead...
Seriously, the answer to the question is that the particle would appear to us to be in both states, not simply one state that we don't know about.
The reason for this is that interference and half-mirror effects actually require the waveform to be the accurate description of reality. This does not, of course, make sense to our large scale interpretation of events.
Roger Penrose (Emperor's New Mind etc) covers this subject well and accessibly I think. He goes on to argue that such inconsistencies between quantum and "real world" events would be resolved by a successful quantum description of gravity... at which point my grasp of physics fails me. i do not know enough about current superstring theory to know if this is really the case, but I am least confident that someone out there will eventually know the answer to this question....