If you create a client from the checkout window and do not click "Save Client" before processing the payment the system does not save the client (Even if you click "Save Client" on the following screen (Where you can print a receipt)).

This can leave an order without a client even when you intended to add a client and in fact clicked "Save Client"

Being able to add a client to an order that doesn't have a client assigned would fix this issue. Making a permission level for this would prevent fraudulently assigning clients for loyalty purposes.