Developer and AI Both Missed a C++ Bug That Only Sanitizers Could Catch

A C++ worker pool was intermittently returning garbage values in release builds, but not in debug builds, pointing to classic undefined behavior triggered by compiler optimization. Both the developer and an AI coding assistant incorrectly diagnosed the issue as a thread synchronization problem and recommended adding mutex locks. The real cause was a dangling reference: a lambda inside a loop was capturing a local variable by reference, but that variable went out of scope before the async task could execute. Running AddressSanitizer and UndefinedBehaviorSanitizer on a minimal reproducer immediately identified the exact line responsible for the stack-use-after-scope error. The fix required just one word — changing the lambda capture from by-reference to by-value — underscoring that sanitizer-based testing, not code review alone, is what reliably exposes lifetime bugs.