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As reported here: http://drupal.org/node/1730678#comment-6435198
This problem occurs when creating new group content using Entityreference Prepopulate. If you're a member of the group and have permission to use the Administrator portion of the complex widget then the group passed via the URL will be set as the default value for both the Default AND the Administrator input elements. This causes an error on save when you have cardinality for the field set to 1.