The security groups feature doesn't fully work in fake mode. If you start fake mode up as a daemon so you can monitor it and then create and delete an instance, you'll see it fails with "Attribute Error: 'FakeSecurityGroups' object has no attribute 'delete'."
When an instance is deleted, the server and other resources are deleted in the reddwarf.taskmanager.models module, which is called from "delete_async" in reddwarf.instance.models . The tail end of delete_async tries to delete the SecurityGroup, which in fake mode isn't totally supported.
The stack trace happens in an async call that has already finished it's work, meaning the tests check that the instance is deleted and move on. If they could check that the security groups were actually gone, this might be caught. However, a better idea might be to wait until Mike Basnight's work on Actions is finished- then the tests could query that the async action finished without errors and catch such bugs.
