False Eyes

The Black group in Diagram 1 has two separate internal empty intersections, but only one of them is an eye (k9), White can play at i12 and make Black "close" the false eye. Then all the Black stones can be captured.

This is a case on the edge of the board. The internal space at g2 is not (yet) an eye. If it is Black's turn a play at f1 will make g2 a false eye. But if White plays first, playing at f1 will ensure two eyes.

In Japanese (and in some English-language books) this situation is called kakeme, which means something like "broken or flawed eye."