Well, we are now into our 8th year of Mechanical Turk and there is now a way to reverse rejected assignments through the user interface. Amazon Mturk Blog Post About Reversing Rejections
Often, new requester will make a mistake and unintentionally reject assignments or grading errors cause work to be rejected. This is a very helpful tool to ensure that you can keep your workers happy if you make a mistake. This used to be a major hassle and required extensive contact with Amazon.
Last year @techlist provided a way to reverse rejections without having to contact Amazon. Now it has been added to the user interface.