United Airlines is in trouble, again.

The Chicago-based carrier has already faced harsh criticism for not allowing two girls in leggings to board a flight in March and the infamous April incident where passenger David Dao was beaten and dragged off an over-sold flight after being randomly selected for removal. There was also another incident that month where a first class passenger was threatened with arrest and downgraded for refusing to give up a double-booked seat.

This time, the airline forced a woman taking a flight from Houston to Boston to hold her 27-month-old son for the duration of the journey, despite her having paid over $1,000 on a ticket for her son.

So what was the reason Shirley Yamauchi was given for having to hold her toddler for nearly four hours during the flight?

The airline somehow double-booked the Hawaiian middle school teacher son's seat, selling a ticket to a passenger traveling on standby.

"It was very shocking. I was confused," Yamauchi told Hawaii TV station KITV. "I told [the passenger], I bought both of these seats. The flight attendant came by, shrugs and says 'flights full.'"

Apparently, the airline had allowed both Yamauchi and the other passenger to board the flight on the same ticket, and Yamauchi told KITV that she was fearful of being separated from her son or dragged off the flight like Dao. "I didn't want to get hurt. I didn't want either of us to get hurt," she told KITV.

In making Yamauchi hold her 25-pound son for a nearly four hour flight, United violated both federal guidelines and, ironically, its own internal policies.

United's website clearly states that children over the age of 2 "are required to have a purchased ticket and occupy a seat." Virtually every other airline shares this policy.

She was also forced to violate safety guidelines, as she wasn't able to put a seatbelt on over the boy, and struggled to hold him. On the FAA's website, the agency advises that parents' arms "aren't capable of holding [a] child securely, especially during unexpected turbulence."

When Yamauchi and her son landed in Boston, finishing an 18-hour journey with multiple legs, she claimed she was unable to get a straight answer about whether she could get a refund, telling CBS that it wasn't until her husband posted pictures online that United finally contacted her with an apology.

In a statement to news outlets, United blamed the double-booking on technical problems and poor training.

The airline told KITV that "on a recent flight from Houston to Boston, we inaccurately scanned the boarding pass of Ms. Yamauchi's son. As a result, her son's seat appeared to be not checked in and staff released his seat to another customer and Ms. Yamauchi held her son for the flight. We deeply apologize to Ms. Yamauchi and her son for this experience. We are refunding her son's ticket and providing a travel voucher. We are also working with our gate staff to prevent this from happening again."

The incident is unlikely to produce the financial hit that the airline's settlement with Dao did, a number that Chinese social media has pegged at $140 million, but is likely much smaller. But ironically, while Yamauchi's son's ticket cost nearly $1,000, the standby passenger reportedly paid only $75.