A: Order numbers get generated right before Payment is collected, due to the fact that some of the payment processors require an Order ID of some kind. If payment collection fails (card gets rejected, someone enters test junk, or in the worst case the online processor is down) then the order will never actually become an order, but the system still thinks that the ID has been used.

This applies to people who use Linkpoint, Authorize.net, PayFlowPro and PayByCheck