The British Airways strike next month is to cause at least five consecutive days of flight cancellations.
BA pilots announced three days of strikes - taking place on 9, 10 and 27 September - in a dispute over pay.
Despite the first strikes being on 9 and 10 September, some customers flying between the 8th and 12th have been told their flights have been cancelled - and to rebook or get a refund.
But some passengers said they had received cancellation emails in error.
After initially being informed their flights had been scrapped, they were then told their flights were going ahead.
Sarah Maxwell, from Belfast, was told by email that her flight from Dublin to Dubai had been axed - but when she got through to customer services they assured her the flight was "100% not cancelled".
BBC entertainment correspondent Lizo Mzimba had a similar experience, being told he had been sent a cancellation email "in error".
"I've lost a whole Saturday morning trying to sort out something it turns out wasn't a problem," he said.
BA is yet to comment on whether it had sent some emails by mistake.
Many customers have also complained that they have been unable to get through to BA to make alternative arrangements.
BA says it carries 145,000 customers every day - with a fleet of more than 280 aircraft - and a BA plane takes off from somewhere in the world every 90 seconds.
BA said in a statement it was "extremely sorry" some of its customers were having difficulties trying to rearrange flights.
"Our teams have been working tirelessly to help as many of our customers as possible, in these unprecedented circumstances," it said.
"Our planning teams have been providing our customers with as many options as possible, as quickly as possible, including a full refund or rebooking to a different date of travel."
Customers received emails late on Friday night and in the early hours of Saturday morning informing them their flight had been cancelled.
Many took to social media to complain that they were unable to rebook via the website or get through on BA's phone lines.
Hi My return flight from LA to Heathrow on 9th has been cancelled. Numerous calls to phone cut off. I have DM'd my details across 90mins ago but had no reply or acknowledgement. Are you still there ?— Andy Keates (@andykeates) August 23, 2019
Abby Deem, 32, from Cambridge said her honeymoon plans had been "ruined" after her business class flight to Mauritius on 9 September was cancelled.
"We've been looking forward to this flight for a year," she said.
"Neither of us have ever had the luxury to travel business class, and after the wedding it seemed the perfect way to start our honeymoon."
She said she felt sick when her fiance Jonathan got a text to say the flight had been cancelled.
They have now booked economy flights with Emirates and they estimate it will cost them an extra £500.
Jennifer Bond, from Manchester, was due to fly to Las Vegas with her fiance Simon to get married but their flights to and from the US (11 and 25 September) have been cancelled.
She said: "Nearly two years of saving up and budgeting relentlessly and this happens three weeks before we fly."
It was "impossible" to get through to BA on the phone, she added, so they have booked new flights with Virgin - costing £700 more than their original flights.
"We're now out of pocket and the time to process a refund is four weeks," she said. "It's disgraceful."
Another customer, Anna Redding, was scheduled to fly to Nairobi with her partner for their honeymoon on 11 September and return on 27 September - when the final strike is scheduled to take place.
They received an email saying their outward flight had been cancelled, and their return flight had been delayed.
She said they had also saved up to upgrade to first class flights as it was a "once in a lifetime" holiday.
They have been unable to get through to BA on the phone and she said the advice is unclear, adding: "Do we try to get another flight with someone else but lose the first class or do we wait just in case but risk not getting any other flights?"
The company's Twitter feed was inundated with messages from frustrated customers, with some saying their cancelled flights were still on sale.
In response to one customer, BA said some flights before and after the strike were "still subject to disruption due to operational reasons, including crew rostering and positioning of aircraft".
Travel expert Simon Calder explained two days of strikes had turned into five successive days of cancellations because BA would not send a flight to, for example, Hong Kong, if a pilot was going to go on strike the next day.
He also said BA has to find customers "an alternative flight on the same day if it possibly can, even if it means buying you a ticket on another airline".
If you are delayed overnight, he said BA has to pay for a hotel and meals.
He added: "The worst thing you can do is take a full refund because then you will be buying another ticket yourself and that could well cost more."
The British Airline Pilots Association (Balpa) said on Friday the strikes were a "last resort" born out of "enormous frustration" with airline management.
Pilots have rejected a pay increase worth 11.5% over three years, which the airline put forward in July.
What can I claim if my flight has been affected by the strikes?
BA advice says you can request a full refund, rebook your flight for another time in the next 355 days, or use the value of your fare to fly to a different destination.
If your flight has been cancelled because airline staff are striking, the the Civil Aviation Authority said, then this would be considered within the airline's control, and therefore you have a legal right to either:
- A full refund, and this includes flights in the same journey that might be from a different airline (for example, an onward or return flight)
- A replacement flight to get to your destination
- Or, if you are part way through your journey and don't want a replacement flight, you are entitled to a flight back to the airport you originally departed from
In some cases, passengers may be entitled to additional cash compensation for the inconvenience - but only if you receive notice that your flight is affected less than 14 days before departure.