Dear Twitter,

If you are going to return different JSON in your RESTful services, please tell your users in advance.

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Versioning and life cycle management of your services is crucial for clients.  Developers of Twitter based applications are quite aware of this.  Once again, Twitter changed the JSON response of some RESTful services at the usual time of 8PM PST.  Of course, it broke our popular Twitter application.   Twitter's RESTful services often change at a whim (not the actual call, but the format of the JSON response)

The cost?  Development time to track down this unanticipated problem.  But mostly, a usability cost (as in potential lost new users) as our application was not properly updating over night until we fixed the issue.
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The moral of the story for all Web Service developers is that the versioning of your services is crucial and should be a part of service deployment from the start.