Have you ever caught your dog barking at nothing in the backyard?

I haven't heard of my dogs doing that yet, however my rotweiler has a tendency to bark at airplanes and sometimes she does try to run after it. I think that she thinks it's a great big toy that's just floating right above her head. However, my other dog did bark at himself when he was a puppy, it was at his reflection. It was so funny, he just couldn't understand that it was himself.