The team that installed some of my displays in a major supermarket decided they would tap into the mains supply ( while live ! ) to the checkouts, on a Friday afternoon.
Each till had a long line of trolleys queued up.
Big bang and flash, it takes half an hour to reboot the system, so all the customers left their trolleys in the queue, and went to another supermarket ....
Luckily it was nothing to do with me, and I have never found out what the costs were to the supermarket ( loss of sales, frozen food thawing in the trolleys etc, etc. )
The most expensive mistake by the team I was part of ( not me though ) was at a NASA tracking station in '67, when one of the guys sent the wrong command to the just launched , first ever , all UK satellite ( Ariel3 ) and instead of making it send back high speed data, it switched from the main transmitter , which was supposed to last years, to the back up transmitter - there was no command to revert to the primary one.
This could have been a multi million booboo, but the satellite continued for its life on the backup, thank heavens.