It's been referenced here at the Reform Club and is often mentioned here and there in the literature. C.S. Lewis was well known as an unbeatable debater until he ran up against Elizabeth Anscombe. The history of the event is that Anscombe won the debate and that C.S. Lewis abandoned apologetics (reasoned defense of Christianity) for other things like children's literature. Victor Reppert, who has written his own book about Lewis, disagrees vigorously with the idea that Lewis abandoned apologetics after the Anscombe debate. Read about it here.
In addition, I think the general thought is that Anscombe beating Lewis in debate represented some triumph of the non-Christian over the Christian. She was actually a serious Catholic who was later in life arrested for pro-life activities in England similar to what Operation Rescue did in the United States. More about her and a confirmation of Victor Reppert's report on Lewis's continued interest in apologetics here at First Things.