In addition to a passport and any necessary visas, most countries in the world have a protocol in place that requires you to provide proof that you'll actually be leaving their country (an onward ticket) in order for you to enter.
But if you intentionally miss the last leg of the journey, your ticket won't be cancelled, as there is nothing left for the airline to cancel. If you skip the first flight, your return flight also gets cancelled. To avoid that, don't ever skip a flight on a return flight.
You can visit the United Kingdom without having to present that you have booked a flight ticket. If you are wondering if you need to show flight tickets for the UK visa, the short answer is no. This is because airline tickets are not a necessity.
Even though booking one-way tickets is usually a great option that offers lots of flexibility, sometimes booking a round-trip ticket is the more cost-effective option. This is because airlines will often price one-way tickets equal to or more than round-trip tickets, especially for premium seats.
Airlines may impose certain sanctions including invoicing an additional amount up to the standard fare of the one-way alternative, deletion of mileage programs, etc. But, the probability of this is minimal, because such a breach can only be identified after the flights have departed.
Proof of onward travel means that airlines and governments want to make sure you are not illegally immigrating to their country, so they need to see proof of a return ticket home. Basically, some countries want to make sure you aren't attempting to move there on a tourist visa and never leave.
Proof of residence. A document showing where you will stay during your stay in Spain. A return ticket. Or confirmation of onward travel if you do not plan to return to your home country after your stay in Spain.
You must arrive in country with proof of sufficient funds and a return airline ticket or other evidence that you will depart Greece if asked. The sufficient funds requirement is often met with credit cards. U.S. official and diplomatic passport holders must obtain a Schengen visa prior to arrival.
Is skiplagging illegal? No, but it's against most airlines' contracts of carriage or the rules people must follow to fly with the airline. American Airlines and Southwest Airlines both put skiplagging first on their lists of prohibited booking practices.