Numerous publications connect wall shear stress quantities to endovascular diseases such as stenosis, aneurism, atherosclerosis plaque formation. The goal of this paper is to discuss the accuracy of the computation of wall shear stress in particular and hemo-dynamic flow in general. We present two benchmark problems: steady flow in carotid bifurcation reconstructed from magnetic resonance imaging, and pulsatile flow in femoral artery reconstructed from ultrasound imaging. In the first benchmark we focus our study on the uncertainties due to the Navier Stokes numerical solver, while in the second we look at the influence of fluid structure interaction with artery wall motion. We will show that even in very simple situations one may expect inaccuracy of the order of 10 to 20 percent on the wall shear stress.