First real complaint about my barcode tool came from someone printing product labels for a shop. The EAN-13s came out visually perfect — crisp bars, correct proportions, human-readable digits underneath. Her handheld scanner buzzed on every single one and refused to read. Root cause: I was encoding whatever digits the caller sent, treating the 13th as just another digit. I assumed valid EAN-13 input. She'd copied a code off a wholesaler sheet where the final digit didn't match what the other twelve implied — and EAN readers recompute that last digit on every scan. No match, no read. No error message, no log. Just a buzz. The most frustrating failure mode there is: silent rejection at the consumer's hardware, invisible at my end. That 13th digit isn't data — it's verification. Take the first 12 digits, weight them alternately 1 and 3 starting from the right, sum the products, and the check digit is whatever makes the total a multiple of 10. Every retail scanner run...