Running plan again without any change forces new resources

[shahkamran]

Any idea why I am receiving forces new resource every time I run plan without any changes?

-/+ module.site.aws_instance.nat (new resource required)
      id:                                          "i-0f4c177afaf802830" => <computed> (forces new resource)
#
#
     security_groups.#:                           "0" => "1" (forces new resource)
     security_groups.2661247649:                  "" => "sg-06347d4a136d0c3d7" (forces new resource)