On 16th August 2007, the Skype peer-to-peer network became unstable and suffered a critical disruption. According to official blog post it was windows update service:
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.
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.
There was also news about some Russian crackers brought down the service. Skype has denied this rumor and assured that no malicious activities were attributed or users' security was in danger, at any point.
This incident clearly provides few hints:
(a) Microsoft dominates PC desktop market.
(b) Application code can bring down entire network, so always consider HA ( High-Availability networking and storage) along with app code
(c) Skype does not blame Microsoft. It was their own code