Official comment
I would start with this handy app that will track key changes made to any of you work items: http://www.clarizen.com/appsmarketplace/item/audit-trail-for-work-items.html
This will track who made the change, when they made the change, and what was changed. By default the app tracks changes to % completion, start date, and due date. It is also possible to configure the app to track changes made to other fields in the system.
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