The metaphorical “what if we killed Hitler before he became Hitler”
In studying history, we are restricted by practicality to study only such things that concretely happened. Surely this leads to something like survivorship bias, so we could be placing undue scientific emphasis on things which were unlikely given material conditions, yet occurred nonetheless.
Therefore some level of speculation is necessary I think, in order to learn from the things which went right due to the non-occurrence of events. Like the eternal dilemma of system admins, the proof of their usefulness is nothing happening, things not breaking, which in turn appears as proof that they were unnecessary in the first place.
Best I can come up with is the handful of averted nuclear deployments during the Cold War, but those are fairly well known.
Not a specific event or policy, more a broad institution of the ancient world (it’s codified in the Bible), but I’d argue that the practice of the debt jubilee averted crises. Both in that the discharging of debts acted as a release valve for the underclasses, but more importantly that they prevented any one debt collector or issuer of debt from amassing too much economic, and ergo political, power. So there was likely a lot of strife, power grabs, violence, etc., that was avoided because of those regular erasure of debts.