This is one we looked into awhile back and found out the authorize.net vendor confirmed it was an issue on their end but they would probably not do anything about it. Basically an issue comes up if someone orders with a payment type other than CC that puts the order on hold and then the person decides to pay with credit card. Even if the CC is successful the order stays as on hold because the plugin doesn't change the order status. They have also seen cases where the person attempts a CC, it fails so its put on hold, they use another card and that is successful but it stays on hold.