Done it dozens of time (over a period of 30 years) and never had a problem. Airlines do not have either the time nor the resources to go chasing relatively small amounts. The sequential part of the deal is more to do with breaking the sequence at the begining or somewhere along a multi-sector trip. This gives them the right to cancel the remaining sectors, which they most definitely do. I would conceed that if an extensive pattern of such activity was flagged, perhaps with a corporate travel Dept., that may give them the incentive to investigate. The fact is that millions of pax cancel or don't show up every day, most for valid reasons. It has happened to me several times for perfectly valid reasons leaving me with the choice of having either to fork out for a horrendously expensive one-way ticket or simply go for the option of purchasing a cheaper return. I pay for my fares out of my own pocket.