Exercise 4.5.2.13. Show that the diagram of $\infty $-categories
\[ \xymatrix@R =50pt@C=50pt{ \{ 0\} \ar [r] \ar [d] & \Delta ^1 \ar [d] \\ \Delta ^1 \ar [r] & \Delta ^1 } \]
is a homotopy pullback square which is not a categorical pullback square.