You will get a red warning that this group does not exist. But nevertheless, you will also see a big box which would incorrectly imply that that group exists and has no permissions. The big box (i.e. the <fieldset> HTML tag) should not be shown if the group is non-existent.
Related Gerrit Patches:
|mediawiki/extensions/CentralAuth : master||List of permissions should not be shown for non-existent global groups|
Well, I for one find this useful. It helps you see former global groups (specially those renamed or deleted). I suggest that this be kept as it is. See, for example, https://meta.wikimedia.org/wiki/Special:GlobalGroupPermissions/Cabal