In Microsoft Office 365 Dedicated/ITAR (vNext), you can't remove the last member of a role group. When you run the
Are you sure you want to perform this action?
Removing the member "User" from the role group "RoleGroup".
[Y] Yes [A] Yes to All [N] No [L] No to All [?] Help (default is "Y"): y
WARNING: An unexpected error has occurred and a Watson dump is being generated: securityPrincipalIdssecurityPrincipalIds
+ CategoryInfo : NotSpecified: (:) [Remove-RoleGroupMember], ArgumentException
+ FullyQualifiedErrorId : System.ArgumentException,Microsoft.Exchange.Management.RbacTasks.RemoveRoleGroupMember
+ PSComputerName : computer.contoso.com