Some forms will change slightly based on what country you have selected on the subsidiary record of the customer used on the invoice.
For instance, the US version doesn't have any support for VAT codes by default, but if I select a UK or EU customer, the form will update to include fields for VAT and related.
I'm guessing the reverse is happening for you? I'm not sure what you mean by "messing up." If the system is getting stuck or something, I'm guessing it's happening when it's trying to switch to the country-specific form.