  For any weakly mixing system $(X, \mathscr{B}, \mu, T)$ and $A, B \in
\mathscr{B}$, there is a density zero set $J_{A,B}$ such that $\mu(A\cap
T^{-n}B)$ converges to $\mu(A)\mu(B)$ for $n \notin J$. In this paper, we study
bounds on the size of this exceptional set $J$. First, we show that, given the
rate of weak mixing, we can find an upper bound on the size of $J$. We use this
property to show the existence of an exceptional set $J$ of an interval
exchange system with either $\left|J \cap [0, n]\right| \le Cn\log^{-a}n$ or
$\left|J \cap [0, n]\right| \le Cn^{1 - \alpha}$, depending on whether or not
the transformation is of rotation class. We also explicitly construct an
exceptional set $J$ for the Chacon transformation and give upper and lower
bounds for its size. More precisely, we show that there is a constant $C>0$
such that for any increasing function $h: \mathbb{R}_+ \to \mathbb{R}_+$
diverging to infinity, there is an exceptional set $J \subseteq \mathbb{N}$
such that $\left|J \cap [0, n]\right|\le Cf(n)$ for every Lebesgue measurable
sets $A, B \subseteq [0,1]$, where $f(n) = (\log n)^{(\log \log n)^2h(n)}$. For
the lower bound, we prove that for any $t>0$, there exists Lebesgue measurable
sets $A, B\subseteq [0,1]$ and $C, N>0$ such that $\left|J_{A,B}\cap
[0,n]\right|\geq C(\log n)^t$ for every $n>N$.
