Once in a while someone will call me with a crack in there windshield from a wiper arm. When they are changing wiper blades the spring loaded arm will snap down on them striking the bottom of the windshield putting a nice sunburst star break. 99% of the time this type of break is not repairable, so it is a good idea to put something like a roll of paper towels between the wiper arm and the windshield this will help prevent this from happening. When I am doing a repair under the blade I always do this.