The cause of today's outage came from within our data centres. Data centres are designed to be redundant: when one system fails (as everything does at one time or another), a parallel system takes over. What was noteworthy about today's outage was the coincidental failure of two parallel systems at nearly the same time.
And that was that. Rawashdeh apologised earnestly in the blog post, noting that Twitter can't pin this to the Olympics or an external bug—just two parallel systems coincidentally going down at the same time. [Twitter]