Illustration of the process of galaxies when they are included twice or not at all in the mock catalogue for two consecutive snapshots. Upper panels: the case when a galaxy is counted twice when constructing a light-cone using different snapshots. The grey region shows the snapshot under consideration. The right panel shows the galaxy j at a previous time, inside the i + 1th shell, showing the direction of movement of the galaxy. Due to this direction of movement and the width of the shell, the galaxy j will appear also inside the ith shell at an earlier time (left panel), and consequently, it is included twice. Lower panels: the case when a galaxy is missing when constructing a light-cone. The right panel shows the galaxy j in a previous time, outside the i + 1th shell, and showing its direction of movement. Due to this situation, galaxy j will also appear outside the ith shell at an earlier time (left panel), resulting in a missing galaxy in both shells.