@JenAshleyDollDesigns: "In other words, the payment method/card is either valid or is not valid." In very simple terms, there are two points where an order may be 'cancelled'. The first is before the order is even accepted - usually an invalid card number or expiration date, insufficient funds, bad security code, etc. You could be having hundreds of those every day and would not even know it. The second is after the order has been accepted but has not yet been fully approved by the payment processor - generally flagged by the payment processor's fraud detection algorithm and showing to you as 'pending'.
However, even an approved transaction can be reversed. For example the card holder may have reported an unauthorized transaction.
This is something that a merchant can and does experience anywhere, even on your own site with your own payment processor.
