  Let $u$ be a nilpotent endomorphism of a finite dimensional
$\mathbb{C}$-vector space. The set ${\mathcal F}_u$ of $u$-stable complete
flags is a projective algebraic variety called a Springer fiber. Its
irreducible components are parameterized by a set of standard tableaux. We
provide three characterizations of the singular components of ${\mathcal F}_u$
in the case $u^2=0$. First, we give the combinatorial description of standard
tableaux corresponding to singular components. Second, we prove that a
component is singular if and only if its Poincar\'e polynomial is not
palindromic. Third, we show that a component is singular when it has too many
intersections of codimension one with other components. Finally, relying on the
second criterion, we infer that, for $u$ general, whenever ${\mathcal F}_u$ has
a singular component, it admits a component whose Poincar\'e polynomial is not
palindromic. This work relies on a previous criterion of singularity for
components of ${\mathcal F}_u$ in the case $u^2=0$ by the first author and on
the description of the $B$-orbit decomposition of orbital varieties of
nilpotent order two by the second author.
