I've avoiding taking one flight of a round trip and the rest of the itinerary wasn't cancelled.

I just boarded the flight that I skipped- making sure to have my boarding pass scanned- and then turned around, told the gate agent that I might not take the flight and that she shouldn't plan on me making it (as I had forgotten something). I then didn't take the flight.

When checking in for the next flight after the one I skipped, there were some technical issues, but the rest of the itinerary wasn't cancelled.

If you do this, be sure to tell the gate agent that you might not be flying- otherwise the passenger numbers vs. boarded numbers will be off, which is a debacle and could delay everyone else.