At approximately 9:00am PDT, Twitter was inaccessible for all web users, and mobile clients were not showing new Tweets. We immediately began to investigate the issue and found that there was a cascading bug in one of our infrastructure components. This wasnt due to a hack or our new office or Euro 2012 or GIF avatars, as some have speculated. We are currently conducting a comprehensive review to ensure that we can avoid this chain of events in the future. In an average 24-hour period, twitter.com has been stable and available to everyone for roughly 23 hours, 59 minutes and 40-ish seconds.”]
Source: https://blog.twitter.com/official/en_us/a/2012/today-s-turbulence-explained.html