  We prove an effective restriction theorem for stable vector bundles $E$ on a
smooth projective variety: $E|_D$ is (semi)stable for all irreducible divisors
$D \in |kH|$ for all $k$ greater than an explicit constant.
  As an application, we show that Mercat's conjecture in any rank greater than
$2$ fails for curves lying on K3 surfaces.
  Our technique is to use wall-crossing with respect to (weak) Bridgeland
stability conditions which we also use to reprove Camere's result on slope
stability of the tangent bundle of $\mathbb{P}^n$ restricted to a K3 surface.
