Foam and ice
Was watching some old films of Saturn I and Saturn V launches, with shots from the many pad-mounted cameras. The rain of ice chunks shaken off the LOX tanks was really cool, and of course did no serious damage to the solid aluminum structures of the rockets. The fragile surface of the Orbiter is what required the foam on the ET to try and prevent just such ice buildup. Usually, it works. But if the foam comes off...