The checksum error is presented when it's an invalid bar code. That means either:
a) it's a made-up number - not recognized as a bar code
b) it doesn't match any of the bar codes we sent to the POS (and expecting the POS to return).
c) Sometimes this occurs the bar code printed on the receipt is not clear enough to scan. This can result in a barcode being read differently than what is actually on a receipt.