Blue thinks he has secured 28 squares of territory because the post at A protects the corner. He’s mistaken — there’s just enough wiggle room for Red to set up shop. How can Red invade and make territory?
Edit: added grid numbers to the diagram — Thanks Herman for kicking my butt to do this!
Update: and once Red invades, what is Blue’s best response?
Bonus: a slightly more cramped variation. Can Red invade and survive here?