Apparently the massive Skype outage last week was caused by...get this..."an abnormally high number of user reboots after a routine software update". That triggered some kind of chain reaction that lead to millions of people being without service for two days:

In an update to users on Skype's Heartbeat blog, employee Villu Arak said the disruption was not because of hackers or any other malicious activity. Instead, he said that the disruption "was triggered by a massive restart of our users' computers across the globe within a very short timeframe as they re-booted after receiving a routine set of patches through Windows Update," Arak wrote.

Microsoft Corp. released its monthly patches last Tuesday, and many computers are set to automatically download and install them. Installation requires a computer restart. "The high number of restarts affected Skype's network resources. This caused a flood of log-in requests, which, combined with the lack of peer-to-peer network resources, prompted a chain reaction that had a critical impact," Arak wrote.

Arak did not blame Microsoft for the troubles and said the outage ultimately rested with Skype. Arak said Skype's network normally has an ability to heal itself in such cases, but a previously unknown glitch in Skype's software prevented that from occurring quickly enough. In a statement, Microsoft described its patch as routine and reiterated that the disruption resulted from a bug in Skype software.