Actually that was the very first thought that went through my mind after reading the summary.

But I have to confess that the idea is not mine; I remember reading about it around a year ago, on this very site, in a comment posted by the most prolific Slashdot member of all. For better or for worse, still no place like /.

The very best comment from the blog:

I fail to see how it matters for this particular locking purpose. Please explain what are the gains of scenario 1 gain over scenario 2 from a malicious user perspective:

1. Mallory makes some changes; another user makes other changes to the same document and saves it before Mallory does. Priory to posting her changes back, Mallory retrieves the new hash, and changes the form value accordingly, thus cleverly escaping your locking mechanism
2. Mallory makes some changes; another user makes other changes to the same document and saves it before Mallory does. Mallory posts the form normally, gets notified that the document has been changed in the meantime and clicks "save anyway"