When I was doing the training I was explaining what a Temporary Creditor (or Vendor for the American readers) was and why I hated them so much. As I was talking I was showing that even through the temporary creditor itself had been deleted, the history for it remained.
It was when I tried to open the Payables Transactions navigation list that I got the below error:
Microsoft Dynamics GP
[Microsoft][SQL Server Native Client 11.0][SQL Server]Cannot insert the value NULL into 'VENDNAME', table 'tempdb.dbo.##181163', column does not allow nulls. INSERT fails.[Microsoft][SQL Server Native Client 11][SQL Server]Cannot insert the value