$ mkdir orig
$ cd orig
$ echo "foo" >> foo.txt
$ git init
Initialized empty Git repository in .git/
$ git add .
$ git commit -m "initial commit"
Created initial commit 6563aaf: initial commit
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 foo.txt
$ cd ..
$ git clone orig copy
Initialized empty Git repository in /Users/david/tmp/copy/.git/
$ cd copy
$ echo "second" >> second.txt
$ git add .
$ git commit -m "added second"
Created commit a984537: added second
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 second.txt
$ git push
Counting objects: 4, done.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 283 bytes, done.
Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
To /Users/david/tmp/orig/.git
   6563aaf..a984537  master -> master
$ cd ../orig
$ git log
commit a9845372d23010b86157dde953c1f74ef6f81701
Author: David Chelimsky <dchelimsky@gmail.com>
Date:   Wed Apr 16 11:27:55 2008 -0400

    added second

commit 6563aafe5d04e0315416a96841e6cfda878567ef
Author: David Chelimsky <dchelimsky@gmail.com>
Date:   Wed Apr 16 11:27:20 2008 -0400

    initial commit
$ cd ../copy
$ git reset --hard 6563aafe5d04e0315416a96841e6cfda878567efHEAD is now at 6563aaf... initial commit
$ git push
To /Users/david/tmp/orig/.git
 ! [rejected]        master -> master (non-fast forward)
error: failed to push to '/Users/david/tmp/orig/.git'
$ git push -f
Total 0 (delta 0), reused 0 (delta 0)
To /Users/david/tmp/orig/.git
 + a984537...6563aaf master -> master (forced update)
$ cd ../orig
$ git log
commit 6563aafe5d04e0315416a96841e6cfda878567ef
Author: David Chelimsky <dchelimsky@gmail.com>
Date:   Wed Apr 16 11:27:20 2008 -0400

    initial commit
$ git fsck --full 6563aafe5d04e0315416a96841e6cfda878567ef
$