We were writing a Unix program to parse transactions from some specialized terminals that read customer invoices and the checks that accompanied them, writing the transactions to digital tape to carry over to the mainframe system. During testing our tapes were compared to tapes generated by the legacy IBM system. Our team lead got a call from the customer liaison *early* on morning saying "Do you realize one of your batches was 5 MILLION DOLLARS SHORT - yes, she was shouting. Turns out that the $5 million transaction was the largest we'd ever tested with so far. All others were less than $999,999. It was my bug - I'd put the sign nybl (half a byte) on top of the most-significant digit of the packed-decimal payment-amount field on the test tape, dropping that digit from the field. Trivial fix - I had just been auditing the relevant code the previous day.