We've seen quite a number of late returnees which delayed our departure but I can't recall where anyone was left behind. One time a lady was so late that they had her roommate pack her bags and were handing them over to their representative at the port when the lady showed.
I notice now that for the shore excursions offered by the cruise lines we frequent they add as one reason to select the line's excursions is that they won't leave you behind due a late return. If I recall correctly it wasn't that long ago when they said there was no guarantee you wouldn't get left if late returning even if you booked through the respective cruise line. Anybody else remember that? If so it must be due to the industries' efforts to detract from the cheaper competitive packages offered by independents at a port.
Would have to be a real downer if one shows at the pier to see their ship "hull down on the horizon" and the next port many days away. Like I posted a while back, we left the Canary Islands for Barbados last fall and it took 8.5 days to get across the pond. If we missed the ship at the last port in the Canaries we would have been toast.