ADO.NET Entity Model: An entity object cannot be referenced by multiple instances of IEntityChangeTracker.
I got this error when creating a new detached object. The issue was that a related object wasn't detached from the ObjectContext, but the detached object referenced an attached one. By detaching the related object when I assigned to the new object, this issue was resolved.
A great thread on this here: http://social.msdn.microsoft.com/forums/en-US/adodotnetentityframework/thread/5ee5db93-f8f3-44ef-8615-5002949bea71/