On the flip side, few things end up like planned. Customers don't even know how their systems work, and only discover the edge cases they had handled in the ol

On the flip side, few things end up like planned. Customers don't even know how their systems work, and only discover the edge cases they had handled in the old system when they discover it suddenly not working in the new system, and so on. When I ask customers "does X ever happen" and they say "no, never", I now assume that means "possibly, but seldom". If I need to make sure, I drill them hard on it. I've been in meetings where a line manager doesn't know that one of the workers they manage performs a critical business operation daily, without which the whole thing would grind to a halt. That's one level up. No wonder those who decide, many levels above that, have no idea and get it wrong.