It's a video that's hard to watch.

A passenger on a United Airlines flight is dragged off a plane because it was overbooked.

It's hard to believe, but according to the US Department of Transportation, an airline can bump you from a flight even if you don't agree to give up your seat.

Airlines have to seek volunteers and offer compensation before they resort to forcing people off a flight.

"It took me six weeks to decide which I flight I wanted to get and it was important to me to get the flight I purchased and looked at," Barbara Blum, who's flying from San Antonio to California, said.

Blum did her homework, arrived early for her flight, even snagged some Whataburger; but she admits, "I am not familiar at all with my rights as a passenger."

Overbooking flights is not illegal and often done by airlines to compensate for those who don't show up.

But as we saw in the United Airlines video (courtesy of Jayse Anspach), when volunteers don't take the offers to give up their seats, airlines can and will kick people off.

Experts said those who paid the lowest fare, or checked-in late are often the targets.

"I'm not aware they can force you off a flight because your seat gets randomly picked, how unfair is that to the paying customer, that's let say booked their flight three months ahead of time," Blum said.

It's unclear why the man on the video was targeted. Policies vary among airlines.

United Airlines said it's investigating the incident and has since issued an apology for the behavior shown in the now viral video.

The best advice to get to the airport as soon as you can and check in as quickly as possible. For more information on your rights as a passenger, click here.