Insight into the accuracy of steady shock-capturing CFD methods is obtained through analysis of a simple problem involving steady transonic flow in a quasi-1D diverging duct. It is proved that the discrete solution error on either side of the shock is $O(h^{n})$ where $n$ is the order of accuracy of the conservative finite volume discretisation. Furthermore, it is shown that provided that $n \ge 2$ then the error in approximating $\int p dx$ is $O(h^{2})$. This result is in contrast to the ge...