Modern cars are very comfortable, and we spend a lot of time in them. Sometimes, especially if you're on a long trip in the front passenger seat, it's tempting to just kick back and put your feet on the dash.
As comfortable as it may be, you should never do it. It's very dangerous. It can turn an otherwise minor car accident into a serious event.
One woman loved to do it, even though her husband thought it was dangerous. She also liked to ride without her seatbelt on. Time and again, her husband warned her. She kept doing what felt comfortable until another car slammed into them at an intersection.
It could have been a minor accident with just bumps and bruises. With her feet up and her seat belt off, though, her legs slammed up and into her face when the airbag went off. She broke her femur and her nose, all at once. On top of that, the angle of the crash snapped a bone in her arm. Even after recovering, she still had a limp when she walked, and she felt sore if she had to stay on her feet for a few hours at a time -- something that is very common in many lines of work.
There's no telling how bad her injuries would or would not have been if she'd had her feet down and her belt on, but it's a safe bet they would have been far less substantial.
Of course, even if you follow this advice and every other safety tip you can find, you can still get hurt when another driver makes a mistake. After a serious car accident, be sure you know how to seek compensation for your injuries if the other driver is at fault.