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Let's say you're about to deploy an application. Said app has been heavily tested by your development team, who have all been infected by unit testing fever. It's also been vetted by your QA group, who spent months spelunking into every crevice of the app. You even had a beta test period with real live users, who dutifully filed bugs and put the application through its paces before finally signing off on the thing. Your application is useful and popular. Your users love it. Your users love you. But over the next week, something curious happens. As people use the application, it gets progressively slower and slower. Soon, the complaints start filtering in. Within a few weeks, the app is well-neigh unusable due to all the insufferable delays it subjects users to-- and your users turn on you. Raise your hand if this has ever happened to a project you've worked on. If I had a buck for every time I've personally seen this, I'd have enough for a nice lunch date. Developers test with tiny toy data sets, assume all is well, and then find out the hard way that everything is fast for small n.