To be fair, it's _unscheduled_ downtime that's not acceptable for a stock exchange. They have scheduled downtimes all the time. For example, as far as I can t

To be fair, it's _unscheduled_ downtime that's not acceptable for a stock exchange. They have scheduled downtimes all the time. For example, as far as I can tell the NYSE is only "up" 9:30am-4pm local time, Monday to Friday, and not on certain holidays. Now this does mean that you have to make very sure that the upgrade you start at 4pm on Friday will be totally done and bug free by 9:30am on Monday, of course. And I agree with your larger point that in a situation like that testing done by someone else in a different configuration is of limited (though nonzero) utility.