Digging through Strobelight and Scuba traces, a performance engineer at Meta filtered on a specific std::vector call and spotted an unexpected deep copy hiding behind the innocent-looking auto keyword. The culprit sat in one of the company’s busiest Ads service paths. Swapping auto for auto&—a single ampersand—eliminated the copy and is projected to free up the equivalent of 15 K servers annually. A reminder that even trillion-line fleets can hinge on one character, and profiling tools still pay for themselves many times over.