Ever since Justin.tv moved in, Crystal Tower Apartments in San Francisco's Russian Hill neighborhood has gotten the reputation as a dorm for 20something startuppers. A recent elevator breakdown, now fixed, left some residents to hoof it all the way up the 12-story building. Justin.tv has moved out, but current residents include the entire staff of email startup Xobni and Web-page builder Weebly, as well as employees of Snipshot and Scribd. The connection? All four companies were backed by entrepreneur Paul Graham's Y Combinator incubator. And more are moving in soon, Weebly CEO David Rusenko says. The proximity is a bonus for the tightly networked group of companies — but the elevator episode should be a sobering reminder to all of them of what happens when your startup has a single point of failure.