title: Github Repo Corruption - Sha1 Collision
body: <p>Yesterday one of my team's checkins corrupted our github repo.  On github, they were showing this error:</p>
  
  <pre><code>$ git fsck
  error: sha1 mismatch 87859f196ec9266badac7b2b03e3397e398cdb18
  
  error: 87859f196ec9266badac7b2b03e3397e398cdb18: object corrupt or missing
  missing blob 87859f196ec9266badac7b2b03e3397e398cdb18
  </code></pre>
  
  <p>When I tried to pull onto a different machine, I got this:</p>
  
  <pre><code>Hyperion:Convoy-clone saalon$ git fsck
  warning in tree 5b7ff7b4ac7039c56e04fc91d0bf1ce5f6b80a67: contains zero-padded file modes
  warning in tree 5db54a0cdcd5775c09365c19c061aff729579209: contains zero-padded file modes
  broken link from    tree 6697c12387f8909cfe7250e9d5854fd6713d25c1
                to    blob 87859f196ec9266badac7b2b03e3397e398cdb18
  dangling tree 144becf61ae14cec34b6af1bd8a0cf4f00d346d1
  missing blob 87859f196ec9266badac7b2b03e3397e398cdb18
  </code></pre>
  
  <p>(I get the zero-padded file warnings on both the offending machine and the second machine I pulled to.  I get the broken link error only on the second machine).</p>
  
  <p>I tracked down the blob to the specific file that's the problem, but after going through the Git FAQ's process on fixing a broken link error, I had no luck.</p>
  
  <p>I went through Github's documentation and found a process to delete the master repo from github and repush from the offending machine.  I tried this, but when I went to re-push the master branch, I got the following error:</p>
  
  <pre><code>fatal: SHA1 COLLISION FOUND WITH 87859f196ec9266badac7b2b03e3397e398cdb18 !
  error: unpack failed: index-pack abnormal exit
  </code></pre>
  
  <p>I've got an open ticket with Github but it's taking them forever to respond. Any idea what the problem might be?  Is there a problem at Github that they need to fix, or is there something I can do to take care of this?</p>