commit gone after merge - how to debug?

Hi,

This looks really weird and I cannot explain why it occurs.

Setup is as follows:
 - origin
 - mirror
 - local clone

Reference repository is origin from where builds are done etc. Parallel to that, we keep a mirror that is synced manually (fetch/merge/push).

I do this from my local clone (which is mostly just tracking origin and mirror, no local branches).

What happened is that after a merge of mirror/master into local master, a commit (that also exists on origin/master) is lost.

Lost as in:
pre-merge:
git log <file modified by commit>
 - commit shown in history
git merge mirror/master
 - no conflicts
git log <file modified by commit>
 - commit NOT shown in file history any more and file does not have this change

Doing git log shows commit as being present in repository history. One interesting point is that one of the parents is previous merge commit of same branches.

Unfortunately, I cannot open up repository for public access, but would appreciate any pointers how to debug this.

git fsck finds some dangling blobs/commits, but no other warnings/errors, I can clone repo just fine, everything seams in order.

How can I debug what the merge is doing?

git version 1.7.12.1 on mac: Darwin 12.2.0 Darwin Kernel Version 12.2.0: Sat Aug 25 00:48:52 PDT 2012; root:xnu-2050.18.24~1/RELEASE_X86_64 x86_64

Regards, Igor

PS. please keep me in CC, I'm not on list