  Let $G$ be a simple, finite, and undirected graph with vertices each given an
initial coloring of either blue or white. Zero forcing on graph $G$ is an
iterative process of forcing its white vertices to become blue after a finite
application of a specified color-change rule. We say that an initial set $S$ of
blue vertices of $G$ is a zero forcing set for $G$ under the specified
color-change rule if a finite number of iterations of zero forcing results to
an updated coloring where all vertices of $G$ are blue. Otherwise, we say that
$S$ is a failed zero forcing set for $G$ under the specified color-change rule.
It is not difficult to see that any subset of a failed zero forcing set is also
failed. Hence, our interest lies on the maximum possible cardinality of a
failed zero forcing set, which we refer to as the failed zero forcing number of
$G$. In this paper, we consider two color-change rules $-$ standard and
positive semidefinite. We compute for the failed zero forcing numbers of
several graph families. Furthermore, under each graph family, we characterize
the graphs $G$ for which the failed zero forcing number is equal to the minimum
rank of $G$.
