Big package gets delivered. Two people working down the delivery, scanning and tagging every item. Both of them scan and tag opposite ends of the same box with

Big package gets delivered. Two people working down the delivery, scanning and tagging every item. Both of them scan and tag opposite ends of the same box with inventory markers. An immutable record is created that says there are two of these things in stock. Someone buys it. It gets checked out. One of the markers gets scanned, an immutable record is created saying there's still one in stock. Physical reality has a way of screwing with all the best laid plans of computer systems. As Pat Helland says: there's no transaction rollback when a forklift drives over a box of eggs.