Microsoft's boss provides some interesting data collected through Internet error reporting:

"About 20 percent of the bugs cause 80 percent of all errors, and - this is stunning to me - one percent of bugs cause half of all errors. With this immensely valuable feedback from our customers, we're now able to prioritize debugging work on our products to achieve the biggest improvement in customers' experience. [...] Already, in Windows XP Service Pack 1, error reporting enabled us to address 29 percent of errors involving the operating system and applications running on it, including a large number of third-party applications. Error reporting helped us to eliminate more than half of all Office XP errors with Office XP Service Pack 2."