Fly Boy:

The practice Avionicsguy has described is a hidden city (I think). There are ways of catching people who do this...

1. They won't let you cancel the first segment, especially segments such as MIA-YYZ in a run MIA-YYZ-HKG, because they too, are aware of the fare differences.

2. Even if you rip out the first flight coupon, you still have the passenger receipt at the back which shows the whole itinary. So when you're checking bags at YYZ check-in, the agent looks at your pax. receipt and says, "Hang on, why didn't you check your bags in MIA?" and then... well... you have to pay the fare difference in full

Sure, there are a lot of opportunities like this all over the place, but a better way of taking advantage of them would be to catch the first flight into the originating city using a separate ticket then continue as you would.

(FYI: I have heard elsewhere on FT that check-in agents are paid to catch people with "hidden-city" tickets... )