Have you ever posted a payables transaction with a document date in a future period that doesn't yet exist in Fiscal Periods setup? No doubt this has happened in almost every company that uses GP. This has been an issue in every version of GP. Why it hasn't been addressed yet is anyone's guess but there is a fix called the 'Document Date Verify Tool' and it's free from Microsoft.
In essence, the tool verifies that the document date falls within an existing period in Fiscal Periods setup. If the period doesn't exist, you can't post the transaction. This is a neat little customization that, in my humble opinion, should be incorporated out-of-the-box in GP.
Contact your GP Partner. They can get the customization for you.