Illustration of underestimated uncertainties for the standardised SDSS system. We summarise the underestimation as half the difference between the 84th and the 16th percentiles of flux differences (normalised by their combined nominal uncertainties) of randomly split sources falling into this apparent G magnitude bin. If uncertainties are correctly estimated, this quantity should be 1, as indicated by the horizontal dashed line. Top panel: nominal uncertainties. Bottom panel: calibrated uncertainties (we highlight the very different y-axis range).