I had the misfortune of having a ticket cancelled because I had missed a leg. I was flying this JFK-SFO(stopover)-HKG-ORD-BDL on UA. So on the SFO-HKG sector, I wanted to return to HK earlier and take SQ instead. So when I was in HK, I checked my itineries online, and found that my itinery was gone (and so was the upgrade). When I call UA, they told me that the entire ticket was cancelled, and cannot be refunded. Neither can I pay a fee to recover that. Even though I learnt my lesson, that was a huge waste of money (HKD10000+ for a now one-way ticket) So never ever skip a leg on a multi-sector journey.