They laughed in 1987 when Ray Saunders insisted a NYSE trading system be able to handle 650K trades a day when an average day was 125K.
Then Black Friday happened and all the other trading systems blew up except for Ray’s.
Seasoned programmers and system designers know to allow for way more than expected when designing a system. A simple example: If the client tells you an invoice line item will never be more than $9,999, then make so it’ll handle line items of $999,999. That way, when the unexpected happens (and it will), the system will be able to handle it.
That’s what Ray planned for. Well done.