A good few years ago I needed to get from London to Los Angeles. I don't remember why but I ended going via San Francisco. After clearing customs and immigration I rechecked my bag and headed for the United flight which left and arrived on time. Stood by the carousel in Los Angeles for a while and nothing appeared. When I went to baggage services I was told it had come down on the previous flight and it was in "that stack" in the hall (and it was).