Sometimes after an operation such as an extrude/ subtract, there will be unconnected geometry. If it is moused over it and other geometry will highlight as if it is all connected; it behaves as a single part.
The vertical airfoil in the graphic below came about through such an operation. I do know how to prevent it from happening. But I am wondering if there is a way to easily remove such geometry after it is produced. "Delete Face" does not ever seem to work. It would make modeling very convenient if it did in such a case.