United Airlines has apologised to a Hawaii teacher who was forced to hold her 2-year-old son on her lap for three and a half hours after an employee gave the toddler's purchased seat to a standby passenger.
Local media reported that Shirley Yamauchi paid almost US$1,000 each for two tickets because children over the age of 2 are required to have their own seat.
She boarded the plane in Houston with her son, Taizo, and they took their seats.
The Kapolei Middle School teacher says a flight attendant came to check if Taizo was present before a standby passenger showed up with a ticket with the toddler's seat number.
Ms Yamauchi says she told a flight attendant about the problem, but the woman just shrugged, said the flight was full, and walked away.
The company later issued an apology.
The incident comes just a couple of months after United became embroiled in controversy after viral videos showed Dr David Dao yanked from his seat.
In response the carrier announced it would offer passengers US$10,000 to give up their seats on overbooked flights as the company attempts to repair its reputation.
United also said it would take actions to reduce overbooking flights and improve customer satisfaction.
"Our goal is to reduce incidents of involuntary denial of boarding to as close to zero as possible and become a more customer-focused airline," the carrier said in April.
Dr Dao, 69, lost two front teeth, suffered a concussion and broken nose being hauled off a Louisville, Kentucky-bound United flight before takeoff from Chicago's O'Hare International Airport, his lawyers said.
* Agencies