Mrs Anna Kwek Rocha once boarded a flight to find someone already in her seat.

It turned out that the seat she was assigned had been double-booked. There were no other available seats as the Emirates flight from Dubai to Singapore, with a stopover in Colombo, was full.

"In the end, I approached the cabin crew (for help). The plane was about to take off, so they had no choice but to give me their seat," said Mrs Kwek Rocha, a Singaporean in her 30s who works in the fashion industry.

She sat in the jump seat located at the side of the plane for the approximately eight-hour journey back to Singapore. Meant for the crew during take-off and landing, it did not come with inflight entertainment and a food tray.

Although she lodged a complaint about her experience with the airline, it did not offer her any compensation, said Mrs Kwek Rocha.

The issue of what airlines do when their flights are overbooked came under scrutiny following a United Airlines fiasco on April 9, when a passenger was dragged off a full plane to make space for airline staff.

What passengers are entitled to

What passengers who are bumped off flights are entitled to in different parts of the world, according to Ms Grace Cheng, co-founder of lifestyle and personal finance website Get.com: SINGAPORE AND AUSTRALIA There is no hard and fast rule. Depending on an airline's overbooking policy, it may include an incentive for volunteers who forgo their seats or compensation for meal, transport or accommodation during the delay. EUROPE Under the law, those who encounter delays above three hours after being put on alternative flights can get cash payments of €250 (S$370) for flight distances below 1,500 km, €400 for distances of 1,500km to 3,500km and €600 for flights of more than 3,500km. Airlines can offer more to sweeten the deal. UNITED STATES Passengers get 200 per cent of their one-way fare to their final destination (up to US$675, which is about S$943), if their rescheduled flight lands •one hour to two hours after their original arrival time on domestic flights, •or one hour to four hours after their original arrival time on international flights. They get 400 per cent of their one-way fare (up to US$1,350) if their rescheduled flight •lands more than two hours after their original arrival time on domestic flights, •lands more than four hours after their original arrival time on international flights, •or if the airline does not make substitute travel arrangements. They get nothing if their rescheduled flight lands within an hour of their original arrival time.

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Selling more tickets than there are seats is a common industry practice, said airlines, which means passengers may get booted off flights.

In Europe and the United States, laws stipulate the compensation passengers who give up their seats are entitled to. They may also give more on top of the mandatory cash amount.

But in Singapore, there are no rules and the incentives are negotiated between the airline and the passenger.

Mrs Lilian Au-Yong, 57, did not bother to bargain for more when Qantas Airlines offered her and her daughter $400 in cash each and a free upgrade from economy to business class for giving up their seats, if they take a flight the next day instead. The offer was made at the check-in counter at Changi Airport, where the pair had arrived 11/2 hours before their Perth-bound flight was due to depart.

It was the eve of Chinese New Year, so the flight was probably overbooked, said Mrs Au-Yong of the incident two years ago.

"I didn't ask the staff too many questions, in case they changed their mind (about the offer)," she added. "They were very polite about it."

The airline agreed to check in the pair's two pieces of luggage first, so they did not have to carry them home and back to the airport the next day.

The postponement also did not disrupt their accommodation plans as they were staying in her daughter's boarding school.

In some happy cases, passengers are bumped up, instead of bumped off. Business development executive Ryan Kuan, 29, and his girlfriend were upgraded from economy to premium economy on a Cathay Pacific flight from Hong Kong to Osaka in 2015.

"It was our connecting flight from Singapore to Osaka," he recalled. "The flight was overbooked as there was an overflow (of passengers) from another flight. So we were upgraded to premium economy.

"It was a good start to our trip," he added.

Transport and travel experts say that while overbooking is common, passengers should know what their rights are.

For instance, said Singapore Management University's transport specialist Terence Fan, passengers should not take up crew seats as Mrs Kwek Rocha did.

"In the event of an accident, it is not clear whose responsibility it is if that particular passenger is hurt," he said. This is because such seats are not meant for passengers.

Meanwhile, passengers like Mrs Au-Yong can try to hold out for more compensation, but run the risk of fellow passengers taking up the initial offer.

Mrs Lilian Au-Yong, 57, did not bother to bargain for more when Qantas Airlines offered her and her daughter $400 in cash each and a free upgrade from economy to business class for giving up their seats, if they take a flight the next day instead.

Still, "if the airline is desperate to get volunteers and no one has volunteered so far, the airline may raise its offers," said Prof Fan.

Every customer has his price, and it is up to the airline to decide how to meet it. Besides free upgrades and cash, airlines can also offer free accommodation and meal vouchers to affected customers, said Ms Alicia Seah, director of marketing communications at travel agency Dynasty Travel.

As for how passengers can minimise the likelihood of being bumped off, she noted that overbooked flights are more common during peak travel periods, such as the year-end, and for popular destinations such as Europe.

And those travelling solo or in pairs are more likely to be selected than larger groups, she added.

To avoid being bumped off, passengers should check in early and arrive at the boarding gate at least 30 minutes before departure.

That said, the experts say that the practice of overbooking flights "varies from one airline to another, from one route to another, and also from one cabin to another".

"On many overbooked flights, no passenger is actually left behind and unable to board the aeroplane," said Prof Fan.

Said Ms Seah: "With advanced technology that can track bookings and allow passengers to check in online, overbooked scenarios have fallen over the past five years."

Passengers can also just say no to being offloaded, and in most cases, the airlines respect that.

Accountant Rudy Chen, 40, was asked to give up his seat on a SilkAir flight bound from an Indonesian city to Singapore during Chinese New Year this year.

The airline offered him a free flight on another airline, but he rejected it as he had a connecting flight to catch.

The airline eventually found another passenger to agree, but the incident left a sour taste with Mr Chen, who is a member of its frequent flyer programme.

He said: "I had a connecting SilkAir flight the same day. What will happen to it if I choose to accept their offer?

"SilkAir could have been more thorough in assessing which passenger gets bumped off."