Two months ago I added barcode symbologies to what had been a plain QR endpoint — code128, EAN-13, UPC-A. My test suite verified the images rendered and that my phone could decode them. It never occurred to me to ask whether the number was valid. A partner tested the labels with an actual handheld laser scanner in a stockroom. Every EAN-13 I'd generated beeped back 'invalid read'. Same image file that my phone camera app decoded without complaint. The root cause is embarrassingly simple:...