Follow-up exploration of the injected mock catalog that most strongly (and incorrectly) excludes ζ spike = 0 in Figure 3. Left: the joint posterior on ζ spike and μ eff from the selected catalog. In general, we find significant correlations between ζ spike and the mean μ eff of the spinning “bulk” population. Mock catalogs that most strongly disfavor ζ spike = 0 are generally those that have the largest inferred μ eff. Middle: the individual events comprising the selected catalog. The event with the largest measured χ eff (and hence the event that most strongly influences μ eff) is highlighted. Right: the solid blue curve shows the updated ζ spike posterior after removing the highlighted high-spin event. Compared to the original ζ spike measurement (dashed black curve), the updated posterior is now more (correctly) consistent with ζ spike = 0.