First off, let me say that I should have read the fine print on the ticket.  Of course, it was an e-ticket, so the fine print must be found, somehow, on a bad web site, but that's neither here nor there.  I booked a round trip from San Francisco to New York on American. 

My plans changed at the last minute as I had a meeting in LA.  My best shot was to fly to LA and on to NY on different airlines and return on my existing return ticket.  The day of my return, I called AA to check on the flight times and lo-and-behold my reservation was gone.  Voided.  Annulled.  I was told that because I hadn't taken my outbound leg, the entire ticket was cancelled with no refund.  There was no phone call (though they had a contact number) and very little sympathy.  Ignorance of arcane and illogical rules, apparently, is no excuse.

I ended up paying for the difference in cost between my reservation and a new one way fare - I got lucky - it was only $75.  AA said that they were "doing me a favor" because technically they didn't have to credit me the money I already spent.

What is ironic is that carriers with great customer service - JetBlue and Southwest for example - would have kept my return leg and credited me the difference!  Logical and useful for travellers whose plans change.

Big carriers take note - the times are changing and you will have to change with them or become obsolete!