Up until now, I've been guilty of putting my feet up on the dashboard when I'm a passenger in our family's car. This video explains why not to. To put it crushingly, crackingly simply: if one has their feet up on the dash and the airbag goes off, it can be fatal in a crash.
Your chances of surviving a wreck are much higher if you're sitting properly with your legs on the ground. The video shows that if your legs are on the dash the airbag can propel them in towards your body, causing severe injury.
Although I love sitting this way because it's super comfy, I'm going to stop for good now.
See Also: Tiny vehicles, big impact: the past, present, and future of the ingenious microcar