The No-show status
If for some reason you are unable to be present on a flight, you must warn the travel agency about it in advance. In the other cases carrier has all rights to cancel all your next ticket segments.
Airlines apply penalties to each passenger who did not show up for the flight. After this all changes in ticket can be made with double penalty.
Besides, the airline also punishes travel agencies, which have issued ticket, if they did not cancel it in time.