  Within the standard electroweak model we carry out an instructive analysis of
the effect of $K^{0}-\bar{K}^{0}$ mixing on $CP$ asymmetries in some weak decay
modes of $D^{\pm}$ and $\stackrel{(-)}{B}$$^{0}_{d}$ mesons. We point out that
a clean signal of $CP$ violation with magnitude of 2Re($\epsilon$) $\approx
3.3\times 10^{-3}$ can manifest in the semileptonic decays $D^{+}\rightarrow
l^{+}\nu^{~}_{l}K_{S}$ ($K_{L}$) vs $D^{-}\rightarrow
l^{-}\bar{\nu}^{~}_{l}K_{S}$ ($K_{L}$). The $CP$ asymmetries are also dominated
by 2Re($\epsilon$) in the two-body nonleptonic decays $D^{\pm} \rightarrow
(\pi^{\pm}, \rho^{\pm}, a^{\pm}_{1})+ (K_{S}, K_{L})$ and in the multi-body
processes $D^{\pm}\rightarrow \pi^{\pm}+n_{0}\pi^{0}+n_{\pm}
(\pi^{\pm}\pi^{\mp})+ (K_{S}, K_{L})$, where $n_{0}$ and $n_{\pm}$ are
integers. It is possible to observe such $CP$-violating signals with about
$10^{7}$ $D^{\pm}$ events at $\tau$-charm factories or hadron machines. Finally
we show that the $CP$ asymmetry induced by Re($\epsilon$) may compete with
those from $B^{0}_{d}-\bar{B}^{0}_{d}$ mixing and final-state interactions in
the semi-inclusive and exclusive decays $B_{d}\rightarrow X(c\bar{c})+(K_{S},
K_{L})$ on the $\Upsilon (4S)$ resonance.
