Skype outage caused by server overload, bug in Windows client
The bug only affected users of version 18.104.22.168 on Windows -- unfortunately, almost 50% of Skype's users happened to be running that version. Included among those were more than a quarter of Skype's supernodes. As those supernodes failed, an increased load was placed on the remaining supernodes. The increased load coupled with a flood of users attempting to reconnect eventually caused Skype's network to collapse under the weight of traffic that was 100 times greater than normal.
In order to prevent future outages, Skype is looking closely at their process for testing and deploying new versions and Rabbe also promised increased investment to boost capacity and reliability.