"Server overloads and a bug in Skype for Windows caused the two-day outage for the net phone firm. Details of what caused the service to be unusable for millions of users prior to Christmas have been posted on the firm's blog. The two events combined to create a cascade of problems that managed to knock out much of the network underpinning the phone service. Skype is assessing how its network is built to stop the problem recurring."

Source: BBC