Sometimes planes are over sold. Like my flight tonight to Madison. I was offered a free round trip plane ticket anywhere in the US if I gave up my seat and would come tomorrow morning. In the past I was always ready to jump on these kinds of deals, but I had an 8am meeting in Madison. But there was a bus that left for Madison and would get me in at midnight or one am instead of 11:00 on the plane. So I opted for the free ticket. But as I was getting the ticket another woman asked if I was going to Madison, she offered me a ride. Pulled into my friend's place at 11:30 pm about the same time I would have had I kept my seat on the plane.
Now I offer you this quick thought before I go to bed. You take the free plane ticket if the opportunity cost is low or someone can drive you to Madison just as quick. In this case I bet I got to the house quicker, so there is such a thing as a free plane ticket.*
*(except for all the time I will waste waiting for that flight to leave)