Twice a month somebody in operations finds a number that doesn't match. An order recorded at €120.00 against a capture of €119.40. Or an order with no payment at all. Or, the one that ruins the afternoon, a refund larger than anything that was ever collected. Then a week goes by. Operations says the payment system is wrong, engineering says the order system is wrong, somebody exports both into a spreadsheet, and the spreadsheet becomes the thing everyone argues about instead of the systems. A de...