Deleted mixed-case groups loitering in JIRA, throwing ImmutableExceptions and causing trouble

To replicate:

• Go to the Studio Group Browser
• Create a group
• Delete the group

The deleted group still displays in the Group Browser. Trying to delete it again will result in the error:

Exception trying to remove group: com.opensymphony.user.ImmutableException

and a stacktrace:

2009-06-30 10:57:51.892058500 2009-06-30 10:57:51,890 TP-Processor1 ERROR [jira.bc.group.DefaultGroupService] Exception trying to remove group: com.opensymphony.user.ImmutableException
2009-06-30 10:57:51.892061500 com.opensymphony.user.ImmutableException
2009-06-30 10:57:51.892062500 at com.opensymphony.user.Group.remove(Group.java:152)
2009-06-30 10:57:51.892063500 at com.atlassian.core.user.GroupUtils.removeGroup(GroupUtils.java:211)
2009-06-30 10:57:51.892065500 at com.atlassian.jira.bc.group.DefaultGroupService.removeGroup(DefaultGroupService.java:806)
2009-06-30 10:57:51.892116500 at com.atlassian.jira.bc.group.DefaultGroupService.delete(DefaultGroupService.java:184)
2009-06-30 10:57:51.892118500 at com.atlassian.jira.web.action.admin.user.DeleteGroup.doExecute(DeleteGroup.java:66)
2009-06-30 10:57:51.892119500 at com.atlassian.jira.web.action.admin.user.StudioDeleteGroup.doExecute(StudioDeleteGroup.java:61)
2009-06-30 10:57:51.892120500 at webwork.action.ActionSupport.execute(ActionSupport.java:153)
2009-06-30 10:57:51.892129500 at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:54)
2009-06-30 10:57:51.892130500 at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:132)

likewise trying to add users to the group fails:

java.lang.UnsupportedOperationException: Could not add user: sysadmin to group: testgroup
at com.atlassian.jira.user.util.UserUtilImpl.addUserToGroup(UserUtilImpl.java:91)
at com.atlassian.jira.user.util.UserUtilImpl.addUserToGroup(UserUtilImpl.java:80)
at com.atlassian.jira.user.util.UserUtilImpl.addUserToGroups(UserUtilImpl.java:108)
at com.atlassian.jira.bc.group.DefaultGroupService.addUsersToGroups(DefaultGroupService.java:319)
at com.atlassian.jira.web.action.admin.user.BulkEditUserGroups.doExecute(BulkEditUserGroups.java:114)
at webwork.action.ActionSupport.execute(ActionSupport.java:153)

The group is actually deleted from Crowd, and restarting JIRA makes it properly disappear.