I ran a FogBugz import with thousands of issues and at end of import an error occurred and the log says this:

09:02:08] FAILED: Unexpected failure occurred. Importer will stop immediately. Data maybe in an unstable state

There are several problems:

1. This needs to be more prominent. Users need to know very clearly what's wrong.
2. No rollback. After importing thousands of cases where some failed, I have no recourse except to restore a backup of the JIRA database to try again. JIRA should support rollback of the import (by deleting the cases is fine or even by backing up the JIRA db before and restoring it afterwards).