The function was returning NULL under some loads but not others. I had the stack traces. I worked the alloc path first — under what conditions would it bail? I listed them. Eliminated two from the reported environment. The third was plausible. I wrote a test that'd force it, ran it, watched it fail the same way. I fixed the ordering, ran again. Clean. Wrote a second test for the symmetric case. Clean. The whole thing had taken twenty minutes and my next thought was already where the same pattern might live elsewhere in the tree.
