My dad worked for a major brewery (You drink in the UK? You've used their product).
One day, the guys doing the rounds delivering to pubs noticed something. Each time they returned to the depot they were given a job sheet, and the job sheet was "computer-optimised" to take account of their route, the best path, time-of-day, traffic, stock, urgency, landlord opening times, etc. So you'd get a really random mix of products to be delivered to a random mix of pubs, all over London. Then you deliver in that order, return to depot, get another job-sheet, load the truck, off you go again, etc.
This day, however, they noticed that there were odd goings on. They were visiting pubs and the landlords were over-keen to take the delivery, and they were delivering into cellars that looked full already.
Turned out that a cleaner had disconnected the expensive computer system that ran the entire company (not the first time... once they did it and the computers poured a million gallons of beer into the sewers...). It hadn't remembered where it had got to, so it still saw a ton of deliveries that had been paid for but still needed to be made, and a reduced stock, and different times of day, so it had recalculated everything and printed out job-sheets for it all over again. But, of course, if it had just been the same sheet with the same deliveries in the same order, someone would have twigged, or the delivery guys would be getting deja vu. Instead, it was essentially randomly mixed-up again, so nobody recognised and different crews did the same job twice without realising. And a London pub landlord isn't going to argue about a second "free" delivery when they already have a cellar-full.
I believe it cost them on the order of millions, mostly because they then had no idea or complete record of who'd had what delivery when, if at all, so they couldn't just assume every job was doubled, or that none of them were... there were landlords complaining that they'd had nothing at all (because they weren't in the initial batch before it was rudely interrupted, and the second batch eventually had a halt called to it while they found out what the problem was), and other landlords hastily shoving kegs out the back door (and onto eBay presumably) so that the brewery didn't know about them - and the delivery drivers were hardly going to dob them in, or at the very least could be bribed with the promise of a few free pints later that evening and a "lost" delivery docket...
Not long after, they got a newer computer system of some description that had some semblance of integration with the actual deliveries going out, and what was signed for. But before that, I'm assured that the power plugs had a big sign put on them for the cleaners NOT to disconnect it to plug in their hoovers....