Retrieving a file at a before a specified commit

Hi all,

I have a commit like this:

    commit 4d77a3cee01db0412956d40875c79f51ac745acc
    tree 3443c9f633114c3bd2e015453a8c55a171e62b53
    parent 340d808ade8a79857bec40770f0eb4f98224c53d
    author ....
    committer .....

which modifies file A/B/C (ie specifically does not add, but changes an existing file in the repo).

I would then like to retrive the version of the file A/B/C before commit 4d77a3cee by using the parent commit 340d808a:

   git show 340d808ade8a79857bec40770f0eb4f98224c53d:A/B/C

which works for most files/commits I try this with, but doesn't work in one particular case.

Questions:

- Is my understanding of the above git command incorrect?
- Is this a corrupt repo? Is there some way to check?
- Is there some explaination of why I can't get the previous version
  of that file?

Appreciate any light that could be shed on this.

Cheers, Erik