Compute $\begin{pmatrix} 4 \\ 5 \\ -1 \end{pmatrix} \times \begin{pmatrix} 4 \\ 5 \\ -1 \end{pmatrix}.$
We have that
\[\begin{pmatrix} 4 \\ 5 \\ -1 \end{pmatrix} \times \begin{pmatrix} 4 \\ 5 \\ -1 \end{pmatrix} = \begin{pmatrix} (5)(-1) - (5)(-1) \\ (4)(-1) - (4)(-1) \\ (4)(5) - (4)(5) \end{pmatrix} = \boxed{\begin{pmatrix} 0 \\ 0 \\ 0 \end{pmatrix}}.\]More generally, the cross product of any vector with itself is the zero vector.