having their card declined

For real. There's a lot of reasons the card could decline. It could be an error anywhere between the bank and the POS. It could be the bank or POS. The card reader could be especially temperamental today because it's a piece of shit and every time they "replace" it, they're just switching out a small collection of units that all have the exact same problem. It could be a fuckup at some network provider (I'm looking at you, Rogers!) that's affecting our ability to process card transactions. On and on and on. I assume all of those things long before it even occurs to me that the problem might be a customer problem.