Originally Posted by Mega Man
The problem isn't always a straight line. The driver's side has a slight funnel to the right (center of the car), and the passenger side to the left (toward the center of the car).
To make this more difficult, the vents have a lip that curves slightly up and out. It does take some strong acceleration to do; something are cars are capable of.
It's just a big annoyance if it happens, and I'm surprised it was designed as it was.
It was not designed for dice or marbles to be put into the vents.