They might cancel the rest of your roundtrip ticket and refuse give you a refund, or they might ignore it for a while and then ban you, or they might do random things like refusing to honor ff miles. There are no rules to protect you and no one to complain to except the airline's customer service.

Care to roll the dice? :-)

People do this a lot. This is a well-known and old issue known as "Hidden City Ticketing" and goes by other names, as well. It is against the typical Contract of Carriage agreement passengers have their airlines, though it's hard to actually enforce: https://travel.stackexchange.com/questions/17984/is-leaving-...

Note that while this tactic can be money-saving and useful in practice, it does have certain downsides, like if you need to check luggage, or if you expect to collect your airline miles and segments (typically requires completing the full itinerary, though not always).

There's a whole website (https://skiplagged.com/) dedicated to it. The airlines do NOT like it. I believe they sued the creator of the website at one point.

In short, you can't check your baggage, they'll probably notice you didn't take the second leg, and if you do it too many times, you might find yourself with a ban.

There was a whole controversy a few years ago around this. See https://skiplagged.com/about . This website automated that approach so that you got a ticket with a few stops for cheaper but you just left at the stop you wanted originally.

The airlines get grumpy. They may not grant you miles, cancel your return trip entirely, etc.

Skiplagged I think helps you figure this sort of thing out: https://skiplagged.com/