San Francisco Mayor Gavin Newsom was elected lieutenant governor back in November, but, unlike November's other statewide winners, he hasn't been sworn in yet. He's remaining in office in San Francisco so he has more control over the selection of his successor as mayor by the board of supervisors there.
But by delaying, Newsom is creating a risk -- albeit a miniscule risk, given Gov. Jerry Brown's good health -- of a constitutional crisis. As Joel Fox explains well in this post at Fox & Hounds Daily.