Some of those customers are indeed clueless. But the machines themselves aren't that great either. Automation and efficiency almost always come at the cost of decreased resiliency. They work great as long as you're on the happy path.

To give a practical example, fresh produce have that 4-digit code to identify them (e.g., 4011 for bananas). But some fresh produce comes in containers that have barcodes on them, like grapes in plastic bags or clamshells. The machine expects the 4-digit code (printed on the container), but the customer expects to be able to scan the barcode just like everything else that comes packaged. The barcode fails to scan, and the customer will hold up the line to have a worker explain the mismatch in expectations. All because the machine designers built in an assumption (for efficiency's sake) that doesn't match the real world, and the machine can't adapt to it on the fly.