Snow!

I hate it when I forget to clean the snow off the roof of my car. After about 15 minutes of driving, the combination of the heat inside the car and the wind on the outside pushes the entire sheet of snow sliding down the rear windshield totally blocking my view out the back. It's stuck like that for the rest of the trip because the measly rear defroster can't do jack and I'm certainly not gonna pull over and get out.