If a Confluence XML space export has it's space key modified according to the instructions at http://confluence.atlassian.com/pages/viewpage.action?pageId=14286901 to have an invalid key, the space export will still import without throwing any failure. A check for space key validity should be performed on all exports and the import should fail.
1) Create a space export
2) Modify references to the key according to http://confluence.atlassian.com/pages/viewpage.action?pageId=169578#HowcanIcopyaspace%3F-method2 so that the key is invalid
3) Import the space
The invalid space cannot be deleted or accessed, so this can only be recovered from by reverting to backup or by manually changing the space key to a valid key in the database.