If you're afraid of clowns, or just not really feeling the "clowns in our cafeteria vibe," then you're probably shit out of luck. But at least you're still in a deluxe cafeteria.
(DISCLOSURE: I ate at that cafeteria once, and it was tasty as hell)
Update: A Twitter rep wrote to tell me the clown visit was part of a fundraising effort by a local San Francisco charity:
With the help of their neighbors at Twitter headquarters along with their friends and family, students of the De Marillac Academy will have the opportunity to see the circus for themselves! On Wednesday, August 14, performers from Ringling Bros. and Barnum & Bailey Presents Built To Amaze! visited Twitter's San Francisco office to entice circus-goers and encourage employee donations for the kids to attend.