But United hasn't said why it didn't respond to the woman's complaint for more than six weeks, and only responded and fixed the problem after being contacted by CBC News.
Experts say the incident also highlights the gamble passengers take when offering to give up their seats in return for a voucher offering a discount off a future flight. They say it shows United is still struggling to fix customer service issues, after some well-publicized mistakes in the past.
Margaret Duffhues, who lives near Waterloo, Ont., was flying from Buffalo, N.Y., to Orlando last December 29, with a connection in Newark, NJ. While waiting on the tarmac in Newark, Duffhues says there was an announcement on the plane's public address system.
"The stewardess came on the flight and said that they needed to get a certain amount of people off the flight because they had overbooked," says Duffhues.
Duffhues says the flight crew offered a voucher for $400 US to anyone who would give up a seat. When there were no volunteers, Duffhues says the crew started to sweeten the deal. "Finally, they offered a $1,000 US voucher," she says.
At that point, Duffhues and three members of her family agreed to give up their seats.
Duffhues says they waited 10 hours in the Newark airport before getting on the next flight. She says they didn't get to Orlando until after 1:30 a.m., which meant they couldn't use one of the day passes they had bought for Disney World.
Duffhues says they figured it was worth it though, because they had their $1,000 vouchers.
In early May, Duffhues went on the United Airlines website to use her voucher to book another flight to Florida over this year's Christmas holidays. But when she entered the voucher's PIN, she says it didn't work. Duffhues says she called United's toll-free customer service number and was told her voucher was not valid.
"They said my voucher was cancelled the actual day it had been issued."
Duffhues says United cancelled the voucher but couldn't tell her why. She says the United customer service representative told her to email United's customer service department, which she did, but she got no response.
United's PR problems
United Airlines has had some high-profile customer service incidents.
In 2009, Canadian musician Dave Carroll tried and failed to get United to pay for a guitar he says was broken during a flight. Frustrated by United's response, Carroll wrote the song "United Breaks Guitars". The YouTube video went viral.
Four days after it was posted, United's stock had fallen 10 per cent. United apologized and asked to use the video to train its employees and help change its corporate culture. But Duffhues says she hasn't seen evidence of that change.
"It's upsetting. I mean, I was doing myself a favour getting off that plane to get the voucher, but I was also doing them a favour." she says. "Then when you do help them out, they're finished with you. They don't care if you get your voucher, they don't care if you're able to book your flight. It's done. They've got what they wanted."
Airline consumer advocates say getting off an overbooked flight in exchange for a voucher can be a crapshoot because flights are so full these days.
"In fact, the U.S. airline industry just announced an 85.5 per cent load factor, which is an all-time record," George Hobica of airfarewatchdog.com says.
"It may take you several days to get to your destination because there may not be seats on that next flight."
Hobica says passengers shouldn't give up their seats without getting a boarding pass for the next flight, "because a promise that you're going to get on the next flight isn't the same as getting a seat assignment." If you do get off the flight, instead of a voucher, Hobica says, passengers can — and should — demand cash.
"Cash is always better than a voucher," Hobica says, "because vouchers are sometimes hard to spend. Sometimes they have restrictions on when they can be used. They also expire within a year. And sometimes vouchers are only applicable to certain fare classes and certain flights."
The voucher Duffhues received lists an expiry date of Dec. 30, 2014. It does not list any other restrictions. Not only that, Duffhues says, all three vouchers given to her family members who gave up their seats on the same flight are valid.
After being contacted by CBC News, a United spokesperson explained they had made a mistake.
The spokesperson said several customers who initially volunteered to get off the plane (and received travel vouchers) ended up staying on the flight. United says it voided those vouchers but, "Unfortunately, we mistakenly included Ms. Duffhues."
United says it has "reached out" to Duffhues to apologize. The airline says it is also sending her a new, valid voucher in the mail, and says "we hope to have the opportunity to welcome her back."
After this experience, though, Duffhues says that in the future, no matter how good the offer might seem, she won't ever volunteer to get off a plane again.
"I'm going to keep my seat."