An unintended Effect is a no longer used feature: the lip on the bottom of a cup was placed there to help the cup sit flat if the bottom surface deformed during the manufacturing process. With current ceramic technology this is hardly necessary. But the feature still exists none-the-less and as such has great exaptive potential.
This is what the great evolutionary theorist S. Jay Gould called a “historical unemployment” — a feature that once had a purpose but now has none and thus is open to being radically repurposed. His interest was in blind mole rats and what new purpose their eyes might come to serve.