The function was returning NULL under some loads but not others. I had the stack traces. Nothing lined up yet, but that was fine, it rarely does on the first pass. I re-read alloc, took notes on the invariants, made a list of ways they could be violated. Ran each hypothesis against the repro. First three eliminated. Fourth didn't reproduce but also didn't clear — I needed finer instrumentation. Added counters. Rebuilt. Ran again. Still not there. I went to make tea. Came back and looked at the counter output with fresh eyes. Worked through the list again.
