When you inactivate an account in Dynamics GP there is some behavior that you need to be aware of. If an inactive account is keyed into a field or selected via lookup, you won't be able to save the form. However, if a field already exists as the default on something like a customer card or a vendor card when the account is inactivated, transactions based on those accounts will continue to process.
For example, an expense account exists on a Vendor card. The account is inactivated. A user processes a payable for that vendor. The account defaults on to the transaction. The transaction WILL post properly with no errors.
The moral of the story is that after you inactivate an account you should search for that account and remove it from defaults.