Slate of Damocles:

We live in a house with a slate roof. You always see flakes of slate around such houses, and it's clear that slates disappear and have to be replaced. But you never hear about people being hit by falling slates. One theory is that they only come off a bit at a time, the bits floating harmlessly to the ground. If you like that theory, you won't like this picture of an entire slate embedded several inches in our yard: