Well, after...Googling...it, there's a little more to the story. It was actually a fight with Oracle about the usual stuff (Java patents), and the email itself was covered by attorney-client privilege, and therefore didn't have to be shown to the other side, but the drafts slipped through by accident because, you know, the subject didn't actually say "Attorney work product - confidential" until almost the final version. And although there is something called a "clawback", the judge ruled that their attempt was too little and too late. "Your honor, it's devastating to my case!"

Basically, Google relied on an algorithm/search engine to screen for privileged documents instead of having lawyers review things the old-fashioned way and they got bit by that.