git-diff not showing changes (corrupt repo?)

Hi,
A small personal git repository has started lying to me about changed 
files. git-diff sometimes tells me that the index has no changes from 
HEAD, while other commands (git-status, at least) seem to tell the 
truth. It is the same after I commit the new changes--at that point, 
"git-diff-tree HEAD^ HEAD -p" spits out a nice patch, but "git-diff 
HEAD^ HEAD" gives nothing.

I have tried git versions 1.5.1.6-1.5.3.3, and they all act the same, so 
I think the repository is corrupt. Does sound familiar to anybody? If 
this is user error (for example, "git-reset --hard HEAD^" on a branch 
that had already been pulled into another branch), I can restore from a 
recent backup and redo some changes, but I would like to help 
troubleshoot this behavior if it is a git bug. Is this a known behavior? 
git-fsck doesn't reveal anything amiss, besides dangling objects. Does 
anybody know anything about this situation or what can cause it?


Example of behavior:

$ git status
# On branch bak_linear-checks3
nothing to commit (working directory clean)
$ echo "test" >> Makefile
$ git status
# On branch bak_linear-checks3
# Changed but not updated:
#   (use "git add <file>..." to update what will be committed)
#
#       modified:   Makefile
#
no changes added to commit (use "git add" and/or "git commit -a")
$ git diff
$

Thanks,
Dan