Bring old clothes and an old suitcase for your travel. When you reach the destination, wear and throw. Finally discard suitcase and return with only carry on bags with no need for the airline to check in any baggage.

Seriously I do this for some of my trips. And it's cathartic.

Only thing that can possibly screw this up is if I go overboard with shopping at my destination. :)

Depends on the wording of clause in contract of carriage but a reasonably good draft will allow CX to charge without having to sue. This seems to have been what BA did also and this is what hotels etc do when they discover that guest broke things in the room or removed certain items. The card holder can dispute the charge of course and there are procedures and a burden of proof to discharge. As for whether it is a penalty, I do not think it is. It will be viewed as the passenger opting to change the route / itinerary to a more expensive one (which is precisely what happens) and therefore a more expensive price is charged. Penalty is when one charges a super-normal price to repel / discourage a customer, a price that is not anywhere else charged. Here, if CX only charges what a normal HKG-JFK return route costs or whatever the passenger actually flies, how can that be a penalty? That is in fact reality!

I guess the potential argument for it being a penalty is that the sum imposed would not be a reflection of the loss suffered by the airline. In fact the opposite, the airline would be saving money through fuel and ground handling. I'd see a voluntary re-route differently - the applicable fare would prevail.

I thought it only cost 100USD, this is no longer the case?

My understanding is that you can only change at the "historical fare" if there is A space available. However, CX have zeroed A/C/D/I inventory if your ticket involves Vietnam.
This means that you will be charged the prevailing rate (F/J), which is much higher.

Are there any automated checks for this type of thing? It seems like if you traveled with a carry-on bag and only used a mobile boarding pass you might be able to fly under the radar.

You would be a 'no-show' for the sector. I guess if the airline wanted to look for a pattern of this for flights HKG-VietNam they could do it, and adjust their handling accordingly. But that's a big 'if' as to whether they will be bothered... unless they want to force more people to cancel.

To clarify a bit, I'm asking because I'm considering ditching my last leg and flying a CX flight to my actual intended destination.

My itinerary is:
JFK-HKG on CX, arrive in the evening
HKG-HAN on KA, depart the next morning

If I book CX flight to my actual indented destination that departs shortly after my arrival on HKG will it be auto-canceled? It doesn't overlap in timing with my next flight, but I'm not sure how things work with CX.

I will be doing a direct turn on my last leg with Vietnam visa and no check-in luggage. Given Dom's experience, I will have to alert CX/KA staff beforehand

CX will be watching mistake tickets very closely. Potentially looking to recoup the bill as much as possible

Does anyone know if inbound-F/outbound-J pax is entitled to F-lounge?

Connecting between short haul J and long haul F (either way) will get you access to the F lounge.

In my experience you had better be able to show onward booking from Vietnam when checking in also.

Transit is only possible if departing to a 3rd country...I believe this has been in place before the whole mistake fare issue. Hence, direct turn on your last leg will not be possible.

Someone wrote about it here..