In the function Import Bills & Invoices, if the income or expense account in the import file is not a valid account, the invoice is posted anyway, posting a one-sided entry (only on the customer/vendor), causing a balance on the Imbalance-XXX account.
Created attachment 373108 [details] Import file to illustrate the bug Using a book with common and business accounts, dd/mm/yyyy date format, vendor 2001. Expense account is Expenses:Does not exist. Import will post the invoice, with a one-sided entry on the vendor account.
Pull request #457 merged into maint, https://github.com/Gnucash/gnucash/pull/457.