When creating a free text invoice out of the template using the "Create invoice by using the default values from"->"Customer account" you can encounter something very odd looking. For example if your free text invoice template had a line with a cost center but the customer record didn't then the free text invoice form will inherit the cost center in the line but when posting the invoice the system would check the customer master data and if there is no cost center then it wouldn't post it but leave it blank.
Unfortunately though the cost center will remain in the line details which is very misleading in the free text invoice form. It would be better if after the systems does its checks that if a line is not posted to a financial dimension that also the information in the form is cleared. I think this would also help in regards to compliance and user acceptance.