Rare B decays are sensitive probes of physics Beyond the Standard Model, and have been in the spotlight for 10 years because of interesting discrepancies between theoretical predictions and experimental measurements. These discrepancies (or "B anomalies") remain to date, and their interpretation has not changed significantly since 2013, despite improved experimental and theoretical analyses over the years. But whether this points unambiguously to physics Beyond the Standard Model depends crucially on one particular QCD effect within the SM, and to our ability to calculate it precisely. In this talk I will dive as deep as I can into this rabbit hole, leading to some technical aspects of great interest.