This is a particularly annoying bug, and relates to the Spotlight user interface in the Finder. (The Spotlight user interface as a whole isn't entirely pleasant to use, even if the underlying technology is awesome.)

If you create a smart folder, and then tell the folder to start updating its contents, the smart folder is all but useless until results stop coming in (which can sometimes be minutes later). The reason? As soon as a new result comes in, the scroll position of the smart folder window gets reset to the top. So if you have more than 20 or so items in the results list of the smart folder, it's going to be a constant tug-of-war between you and the Finder: you'll start browsing through the results list by scrolling, and the Finder will keep saying, "No! Absolutely not! Let me finish before you can do anything!" Given that results sometimes stream in fast and furious, it's the Finder that usually wins. Filed: 4499374.

Blah. Not that I really use smart folders anyway. This whole deal about folders and hierarchy being eliminated by technology like Spotlight is dumb -- a hierarchical system allows you to find what you want much faster, because you don't have to pore through a bunch of different results.