On August 16, 2007 the Skype peer-to-peer network became unstable and suffered a critical disruption. This was triggered by a massive restart by the users’ computers across the globe within a very short timeframe as they re-booted after receiving a routine set of patches through Windows Update.

This high number of restarts and log-in requests combined with the lack of peer-to-peer network resources caused a chain reaction that had a critical impact. Skype’s peer-to-peer network was designed with the ability to self-heal, but this event brought about an unseen software bug within the network resource allocation algorithm which disabled the self-healing function. 

The issue has been identified within Skype and it has been confirmed that no malicious activities were attributed and security was not at risk.  Over the past four years, Skype has provided a technically resilient communications tool to millions of people worldwide. The company is very proud of that and appreciates their supportive community of users.

(Source)