Currently when overwriting a spec the target id's change. This isn't that much of an issue on it's own as it just means that more topics may be updated than necessary. However coupled with BZ#1092170 which overwrites topics, then it might change the target id for shared topics. This means that the topic would then work for the updated spec but not for the original one referencing it (although the original should probably be referencing the topic at it's specific revision).