Model Context Protocol (MCP) servers that work perfectly in development can fail intermittently once they are deployed across multiple replicas behind a load balancer. The failure mode is a stream of "session not found" errors that appear at random, and the cause is a mismatch between how certain MCP transports hold session state and how load balancers distribute requests. This article explains why the problem occurs, when it applies, and a concrete pattern for solving it using a shared session store.The problem is easy to miss in early development because it only appears once there is more than one server instance. A single-instance deployment holds every session in local memory, so every request naturally finds its session. Add replicas, and that assumption quietly breaks.