Rain hitting your windshield can mean dicey road conditions and traffic snags. According to XKCD's What If, the force of those drops hitting your windshield can also destroy your car's windshield. Lesson learned: don't drive at speeds of Mach 1.5 in a downpour if you want your glass to survive.
As XKCD's Randall Munroe explains, at Mach 1.5 (or nearly 1609.3km/h), the droplets hitting your car's windshield will smoosh out into sideways-aimed water jets doing an astounding Mach 18. The force of these jets will simply erode your windshield glass -- meaning those 1600km/h droplets will now be hitting your face. Ouch.
So, slow the hell down when you're driving in the rain. Keep it at, say, a sensible 750 MPH. Your glass will thank you. [xkcd]