I can understand that it would take time for Panama, Yahoo's new text-ad system, to show up in better revenue numbers. But here's what's less easily explained: how on earth did Terry Semel mismanage analysts' expectations so badly? Less than a month ago, he was talking up the potential of Yahoo's overhaul, which was supposed to place more lucrative ads against search results: "I'm totally all smiles," Semel told Advertising Age. "We are very excited and very happy and I'm smiling broadly." To be sure, the early numbers could have been misleading, like exit polls on election day. But Terry Semel's great management virtue is supposed to be his unflappability. By talking up the results of Panama, prematurely, the Yahoo CEO is behaving like he's under pressure — and eminently flappable.