Pareto set with Cobb-Douglas and Leontief preferences
The bottom right origin is actually not in the Pareto set.  At that point, $(x_A,y_A)=(8,0)$, so $U_A(x_A,y_A)=0$.  Similarly, $(x_B,y_B)=(0,4)$, so $U_B(x_B,y_B)=0$.

As an example, $B$ could give one unit of $y$ to $A$ and thereby raise $A$'s utility to 8 without hurting $B$'s own utility (which would remain zero).  As a matter of fact, any allocation other than the top left origin is a Pareto improvement over the bottom right origin.