This case-study-driven talk uncovers how small, often overlooked design decisions can lead to significant changes in service traffic and customer experience. We will discuss how "design drifts" in internal service calls can quietly increase traffic, cost, and risk. I will share a production case study where reclassifying requests, moving lookups to authoritative domains, and tightening contracts reduced total traffic by approximately 92%. This session demonstrates how early design choices ripple through a system, shaping performance and user impact in ways that may not be visible until much later.