git push failed

With git built from master (v1.6.6.1-436-gaba7dea), the following commands (run in a script):

  git commit --amend -m "Update index.html" index.html
  git push erlang +gh-pages

failed like this:

Counting objects: 4, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 3.85 KiB, done.
Total 3 (delta 0), reused 0 (delta 0)
error: Ref refs/heads/gh-pages is at
db10c045e068f12aa2f78377fab31e35a8443ca4 but expected
054136d96494979b42524a4dc08ef643bb54cc0e
error: failed to lock refs/heads/gh-pages
To git@erlang_github.com:erlang/otp.git
 ! [remote rejected] gh-pages -> gh-pages (failed to lock)
error: failed to push some refs to 'git@erlang_github.com:erlang/otp.git'

Unfortunately (from the point of view of finding the bug), the problem is not reproducible. When I ran my script two more times, it worked fine.

We all know how difficult it is to investigate non-reproducible bugs, but perhaps someone familiar with the code might have some idea about what it could be.

Info about my system:

Mac OS X 10.2.6 (8 cores) gcc-4.2.1