Bias caused by a popular weighting scheme

It is reported how the popular <i>SHELXL</i>-like weighting scheme leads to bias in residuals when the standard uncertainty (s.u.) values of the observed intensities are underestimated, which appears to be the case very frequently. An underestimation of the s.u. values by a factor of 5 destroys the symmetry of the residuals with respect to zero and leads to an average increase in <i>U</i> _equiv of 3.35% in a simulation.