A rock gets kicked up and hits your windshield. The next thing you check is to make sure your windshield didn't get cracked. Sometimes you're lucky and the rock hits at such an angle that there is no crack. According to yourmechanic.com, you can be ticketed for having a cracked windshield. A bit vague, but the moral of the story is that if you do have a cracked windshield, you need to get it fixed.