Twenty-some years ago, I worked as a programmer for a large public utility.
One day, a customer with a $42 monthly "budget" bill was going out of town for a couple months and knew that her overall use would be lower than the prescribed amount. Before leaving, she went to her local office and asked if the clerk could override the preset payment. Our system allowed local clerks latitude to make adjustments, so the clerk tabbed through the screen fields to the bill amount and typed in "35."
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Back in those pre-Windows days, the field size on the screen was much wider than the two digits needed for most bills, and instead of clearing out the old amount, the clerk typed "35" on the left side of the field and accidently left "42" on the right side. Instead of replacing 42 with 35, the computer filled in 0s between them, and the customer was sent a bill for $3,500,042.
The customer raised a ruckus, even though we quickly corrected the problem. This $3.5 million residential utility bill even made the front page of the local paper.
Perhaps we should have all gotten a good laugh out of it, but the "top floor" guys didn't find it amusing. Our department was given orders to fix the program and make sure it never happened again.
Fortunately for me, I wasn't in charge of the billing system, but I was in charge of the bill printing subsystem -- basically a 10,000-line Cobol report program. The solution assigned to me was to code the program to write and send a new daily report that listed the five largest bills for each revenue class that had been printed overnight. I got a copy, my manager got a copy, and several copies went "somewhere else." If we saw any suspicious bills, we were to go to the mailroom and pull the physical document before it was mailed.