Today I learned that when you buy a business class seat on Delta they have the option to move you to coach, without notification, with no recourse. 

They offer to refund the difference in price. But I didn't think I was buying an option to fly business class. I thought when I made the reservation, including a seat assignment, that I was not playing the lottery, that they would not give the seat to someone who paid more.

Their excuse: It was in the fine print that I agreed to when I purchased the ticket.

This is why I hate Delta.