Sales orders can now be "uncompleted"16 Jun 2011
We've now added the option to revert a completed order back to the pick & pack state. You'll find a link that does just that beneath the status controls at the top of the order details page:
We've also added similar functionality to the invoice workflow - you can now revert an invoice back to the 'unsent' state.
UPDATE: You can also now revert invoices back to their unsent state.