Using standalone Crowd as an external directory (internal directory disabled):

• create a user (the 'original' user),
• add the user to some groups,
• delete the user,
• create a new user with same username,
• remove them from all groups,
• restart JIRA.

After JIRA restarts, the user will be in all the groups the original user was in, and it will be impossible to remove them from any of these groups.