Wrong. That is exactly what will happen.

If you must miss the first return leg, you will be well advised to contact the airline to change the itinerary. There will undoubtedly be a change fee but it will be better than arriving at the airport to find you have no valid ticket. In that case you would have to pay walk-up prices for the one-way flight to your destination.

This situation is discussed many, many times in the Travel Buzz section of Flyertalk, under the term "hidden city" ticketing.