It's perfectly possible she was just promoted above her level of competence (Peter Principle in action). She may have been fantastically effective and successful at her job at Google (I don't know either way), but when trying her hand at the CEO job, it turned out it didn't match her skills. Not to mention Yahoo! wasn't exactly in great shape when she took the job; IMO the board screwed up (possibly intentionally): they needed someone with experience bringing a company back from the brink of failure, not someone who'd never been a CEO before.