I have not heard of anything like that cruise-related, but I have had friends book hotel rooms through Priceline and then show up to the hotel to find out there is no room at the inn. (
) In both instances, Priceline moved them to a different hotel. And, in both instances, the hotels were supposedly not as good as what they had originally booked. I don't know if they really had any recourse since they did get a room for the same price.
You mentioned that you were leary of dealing with Priceline or Carnival again. I would be leary of the Priceline thing, but not Carnival. The cruise was a service being offered by a 3rd party, not by the cruiseline, so I wouldn't really blame Carnival in this instance. . . .