• Twitter's issues started at about 12 PM EST yesterday, with web inaccessible for all users and mobile not showing new tweets
• The troubles were due to a cascading bug, which is "a bug with an effect that isn’t confined to a particular software element, but rather its effect 'cascades' into other elements as well."
• Blog post was contrite and well written, but also touted Twitter's recent lack of issues, citing up time of at least 99.96% and often 99.99%

From the article:

Not how we wanted today to go. At approximately 9:00am PDT, we discovered that 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 wasn’t due to a hack or our new office or Euro 2012 or GIF avatars, as some have speculated today. A “cascading bug” is a bug with an effect that isn’t confined to a particular software element, but rather its effect “cascades” into other elements as well. One of the characteristics of such a bug is that it can have a significant impact on all users, worldwide, which was the case today. As soon as we discovered it, we...