I was hit with the "one open jaw" when I did a OWExplorer on miles last summer. It's a real rule. Another rule that hurt me was that you have to start and end in the same city...I was going to start in BOS and end in ORD as we were also moving to Chicago at the time and didn't need/want to end up in ORD.

The thing that really threw me off is that mileage is measured point to point, even if you're connecting. So, I had worked out a really cool itinerary for the 25,000 mile award, eeking out every last mile. Then I found out that we could have, for example, done TLV-FCO on BA thru London, and have it only count as if the miles were nonstop. If I had known that, our itinerary would probably have been fairly different...but I planned it based on OneWorld carriers and routes.

Our routing for the 25K ticket was BOS-TLV (via LHR), TLV-BCN, BCN-FCO, FCO-BKK (via HKG), open jaw (purchased BKK-SYD), SYD-CNS, CNS-BOS (via NRT, ORD). This made it easy to solve the end in ORD problem -- just got off at ORD and went home rather than take the connecting flight

I posted a trip report at http://www.flyertalk.com/forum/Forum81/HTML/001118.html