Let $V$ be a vector space over an algebraically closed field $K$ and let $f:V\to V$ be an endomorphism. If $V$ is finite-dimensional, we know that the characteristic polynomial $\chi_f$ has a zero $\lambda$, which is an eigenvalue of $f$. However, does a similar argument hold if $\dim V=\infty$?
The answer is no. Consider the space of infinite real sequences and let $T(x)=(0,x_1,x_2,\dots)$. Suppose $T(x)=\lambda x$.
If $\lambda=0$, then $x$ has to be zero, so $0$ is not an eigenvalue.
If $\lambda \neq 0$, then $0=\lambda x_1$, so $x_1=0$, but then $x_1=0=\lambda x_2$. Continuing by induction we again conclude that $x=0$. So $\lambda$ is not an eigenvalue either.
So $T$ has no eigenvalues.
Note that there is no contradiction with the finite dimensional case, because the finite dimensional analogue of this "forward shift" operator must necessarily "forget" about $x_n$, which means that $(0,0,\dots,1)$ will be an eigenvector with eigenvalue zero.