Yep, don't ever book the outbound and return on the same itinerary if you are using this trick. Most airlines ( other than Southwest ) will void everything else on the itinerary once they detect it.

It's also one reason why low-cost carriers like Easyjet and Ryanair have thos low operating costs, they only offer point to point flights so there is no hidden-city option and no need to pay for a department to scan for it...