OK to the poster who said they have never seen someone left behind, I have, it does happen. I've been on 20+ cruises and I've seen people left at a port in Alaska, Venezuela (very serious), Southampton, Key West.
I have to mention all of these were not in the last 10 yrs. and Venezuela involved cruise line employees (2) who should have known better. They were told to make their way to LA to pick up their stuff off the ship and last pay check.
I was on the bus waiting for the 2, and it got serious because of the tide situation. Because of the delay, tugs were used to get us out. It was rather scarey. We had to eat with the late seating dinner as we were back over 2 hrs. late.