I was on a UA flight a week ago where the passenger took his "emotional support animal" out of the small carrier (fit under seat). He was asked twice to put it back in because, while the animal can ride in the cabin, it cannot be removed from the carrier. He refused both times until the Captain came on the PA to let the whole cabin know that he was turning the plane back to Miami (we were over the Gulf enroute to Costa Rica) if the passenger did not comply. After a few scowling looks, he did it.