This is a version-upgrade failure mode that's common enough to deserve its own name — walked through the way it actually surfaced, not summarized after the fact. Python 3.15 landed in October 2026 with a genuinely exciting number attached: an upgraded JIT compiler running meaningfully faster, double digits on some platforms. We run a data pipeline that's CPU-bound enough that a free double-digit speedup was worth bumping the version for on its own. We upgraded a staging worker, ran the usual ...