ServiceStack/ServiceStack.Text — pull request #209 by franklinwise (2012-09-30 03:02:06+00:00)
Title: Bug Fix - Deserialize JSON with Type Info in nonstandard position causes half deserialized objects 

If the TypeInfo on a Polymorphic JSON is not listed as the first item in the JSON object, the object will create a new instance part way through deserializing an object causing a partially deserialized object without any error.

Backbone.js will reorder the JSON properties causing this bug.

The behavior has been changed to ignore type info if an instance has already been created. I could see creating an option to throw an exception in this case as part of a config setting. But for now, just allows the existing instance to deserialize.