"tickets which the airlines have offered to sell to people traveling from point A to point B, and under terms which expressly prohibit jumping off at point C. Terms that you generally have to check a box saying you're agreeing to."
Pjeby, the Austrian Airlines conditions of carriage are 21 pages long, and as far as I know that is not untypical. Yes, you have to check a box saying you have read the terms and agree with them, but I think that pretty much everyone knows that everyone checks this box without reading them, and certainly without actually agreeing to them, since they do not know the terms.
And unless you read the terms yourself when you buy a ticket, which I strongly suspect that you do not, how would you know whether or not getting off at point C was prohibited by the terms?
In fact, the Austrian Airlines terms of service contain this:
"3.3.3. If you do not use the flight coupons in the sequence shown with the intention to circumvent the fare system, we will charge you the applicable price for the actual sequence of transportation you intended to take. This will be done by determining the fare you would have had to pay on the day of reserving your actual sequence of transportation. This may be higher than the originally paid cost of the flight. The cheapest available price in your booking class will be used to recalculate the changed sequence of transportation. If the booking class originally booked by you is not available for the changed routing on the day of making the reservation, the cheapest available booking class will be used to recalculate the changed sequence of transportation. Any taxes and charges for the unused flight coupon will be deducted. Please note that we are entitled to make carriage dependent on having paid the difference in price."
Note that you are not agreeing to ACTUALLY TAKE the sequence of flights you purchase. You are agreeing to be charged for the actual sequence, so that e.g. you will have no valid complaint to a credit card company when they charge you the fare difference for getting off in London instead of going on to New York.
So if it is simply a question to keeping to your agreements, if you get off in London after purchasing a ticket from Vienna to New York via London, you are not breaking your agreement. Instead, the airline will simply be failing to carry out its threat if it fails to charge you any addition price difference for the Vienna - London ticket as opposed to the Vienna - New York. And if they do, that is more the airline failing to keep its side of the agreement, then you failing to keep yours.
I was going to wait to post this for reasons, but realized that was pretty dumb when the difference of a few weeks could literally save people hundreds, if not thousands of collective dollars.
If you fly regularly (or at all), you may already know about this method of saving money. The method is quite simple: instead of buying a round-trip ticket from the airline or reseller, you hunt down much cheaper one-way flights with layovers at your destination and/or your point of origin. Skiplagged is a service that will do this automatically for you, and has been in the news recently because the creator was sued by United Airlines and Orbitz. While Skiplagged will allow you to click-through to purchase the one-way ticket to your destination, they have broken or disabled the functionality of the redirect to the one-way ticket back (possibly in order to raise more funds for their legal defense). However, finding the return flight manually is fairly easy as the provide all the information to filter for it on other websites (time, airline, etc). I personally have benefited from this - I am flying to Texas from Southern California soon, and instead of a round-trip ticket which would cost me about $450, I spent ~$180 on two one-way tickets (with the return flight being the "layover" at my point-of-origin). These are, perhaps, larger than usual savings; I think 20-25% is more common, but even then it's a fairly significant amount of money.
Relevant warnings by gwillen:
Additionally, you should do all of your airline/hotel/etc shopping using whatever private browsing mode your web browser has. This will often let you purchase the exact same product for a cheaper price.
That is all.