Let \(I=[0,1)\) , \(-1<\lambda <1\) and \(f:I\rightarrow I\) be a piecewise \(\lambda \) -affine map of the interval I. The exceptional set \(\mathcal {E}_f\) of f is the set of parameters \(\delta \in \mathbb {R}\) such that \(R_\delta \circ f\) is not asymptotically periodic, where \(R_\delta :I\rightarrow I\) is the rotation of angle \(\delta \) . In this paper we prove that \(\mathcal {E}_f\) has zero Hausdorff dimension. We derive this result from a more general theorem concerning piecewise Lipschitz contractions on \(\mathbb {R}\) that has independent interest.