Here are two different ways it can occur.
Have a look at the Blue L. It is often very tempting to wrap our piece around the corner of the opponent's piece like this, because we're really thinking about killing corners. But this is overkill.
The idea is good, but it's important to remember that there is only one square we need to play in to kill the Green corner, not 3.
This Blue L could be flipped as shown here, which is a much stronger move
because it creates more corners and smaller leaks.
Here is another often-seen situation, taken from a different game.
Red has played the F in order to halt yellow's advance, creating a 2-hole.
But red has "over-blocked" ... the red F need not touch the yellow piece ... yellow already cannot play there.
This placement also creates an L4-leak on the other side of the F.
What a difference a simple rotation makes!
This placement of the F creates the same 2-hole
(even if it looks open at the top left),
gives Red more choices of plays going upwards,
and doesn't create that L4-leak. Instead it sets up
a second 2-hole, which is a much more effective block.
