This two-experiment study aims to investigate the role of expertise in offside decision making (Experiment 1) and the effect of perceptual-cognitive training (Experiment 2). In Experiment 1, a video-based offside decision-making task followed by a frame recognition task demonstrated a bias toward flag errors and a forward memory shift for less-successful elite-standard assistant referees that is in line with the predictions from the flash-lag effect. In Experiment 2, an offside decision-making training program demonstrated a substantial progress from pre- to posttest for response accuracy, but not for accuracy of memory in the frame recognition task. In both experiments, no differences were found for visual scan patterns. First, these results suggest that less-successful elite-standard assistant referees are more affected by the flash-lag effect. Second, an off-field perceptual-cognitive training program can help assistant referees to deal with the perceptual consequences of the flash-lag illusion and to readjust their decision-making process accordingly.