Margen
You buy a Madrid–Rome–Madrid flight for 180 euros. Something comes up, you decide to skip the outbound and just show up for the return. When you reach the desk, the ticket no longer exists.
This is not a system failure: it is written down. The general conditions of carriage of almost every airline state that flight coupons must be used in order. If you do not fly one leg, the airline may cancel the following ones without refunding anything. In 2018 Lufthansa took a passenger to the Berlin courts for getting off at the stopover to pay less, and in 2019 it won on appeal.
What to do, in this order:
1. If you are going to miss the outbound, call before departure time and change the ticket. There is almost always a change fee, and it is almost always cheaper than losing the entire return.
2. If you are already too late, claim back the airport taxes and charges. That part is refunded even when the fare is not, because you never went through the airport. You have to ask in writing, and many airlines charge an admin fee.
3. If you buy two separate one-way tickets instead of a return, none of this applies: they are separate contracts. It usually costs a little more, and it is the cheap way to buy peace of mind.
And one that is written nowhere: read the conditions before buying, not after. The word you are looking for is «coupons» or «legs».