RCCL cancelled about a full month of cruises on Empress this spring when they were bringing her back into the fleet and her dry dock to do so ran way over. And reports from people on the first few cruises that sailed indicated that she could have used more time. That's an extreme example, but the possibility does exist that if there are major issues they could cancel the cruises.
It's been a while, but I think I remember the Magic had some pretty bad kinks coming back off her dry dock. I don't know if anything was cancelled, but it might have been.
eta: Thank you Shmoo for the info on the Magic. I couldn't remember if there were cancelled cruises or just "issues".