Staring down the tunnel of the AeroTrain connecting the terminals at Washington-Dulles International Airport (IAD) where this story begins (and, sadly, ends).
The popular travel blog One Mile at a Time published this piece last month, I’m Not Sure What To Make Of This Travel Horror Story, that recounted a sad incident in which a couple (one Croatian and one American citizen) about to board a Turkish Airlines flight on a Washington, DC - Istanbul - Venice route was turned back to check-in and missed their entire vacation due to "travel document" issues. I'll let you read the details for yourself, but will seize this moment to highlight the perils of getting too creative with your travel itinerary. This is particularly relevant to those of us who frequent off the beaten path places such as (ahem) wine country...
In this case, the couple planned to skip the leg of their journey that would have taken them from Istanbul to Venice, instead opting to catch a separately booked flight from Istanbul to Croatia. It appears that this was the source of their issue. I know that it can be tempting to book multi-leg flights with the intention of skipping out partway through, but this is more than technically a violation of your contract with the airline. All later legs booked on that same ticket are cancelled the moment you skip any one leg.
Consider a simpler, but similar scenario…
You've booked a roundtrip flight for the weekend, but it's looking like you won't be able to leave work in time to make your departing flight. Your first instinct might be to just book a new one way flight to your destination later in the day, and then use your original return flight to come home a few days later. Unfortunately, missing the outbound flight in your round trip will cause the return flight to be cancelled. You'd get to your destination on your new one-way, only to later discover that your return has been cancelled. Instead, you've really no option other than to contact the airline and pay any fare difference between your original flight and the new one (plus, often, a change fee).
Had this couple made it to Istanbul and then skipped the Istanbul to Venice leg, they'd have learned later that their flight returning to the United States had been cancelled as a result. Same concept as in our scenario above, just with a few more legs thrown in.
This is why I often book one way flights. In other words, I book my outbound flight on one reservation, and I book my return flight on a completely separate ticket. If something happens causing me to miss the former, I'll still be able to find another way to my destination without any impact to the latter. This is often a reasonable strategy when flying domestically within the United States, but international one ways can be excessively more expensive than booking the same journey as a round trip.
The original article mentions something else, a fact which is likely the basis of why this couple was snarled before even taking off. Ducking out on the Istanbul to Venice leg meant that they'd need to retrieve their checked bags at Istanbul, which by extension likely means they would have had to clear customs and technically "enter" Turkey, an action for which they almost certainly did not have a visa (because they were not technically planning to travel to Turkey). This concept does not apply when remaining behind the customs wall transiting an international airport on a layover, but does apply when having to clear through customs to (in this case) retrieve a checked bag.
So while I have a lot of sympathy for these folks whose vacation was wrecked by a technicality, may their experience serve as a cautionary tale for others thinking of doing the same. At the very least, you will find certain value in researching the implications of particular scenario the next time your travel plans call for itinerary voodoo. The alternative is rather heartbreaking.