When you load test a Node service, how do you make sure the numbers aren't lying to you?

There's a bunch of ways a load test can trick you. First, warm up before you measure, because the first requests hit cold caches and code that hasn't been optimized yet, so the early numbers don't represent steady state. You want realistic traffic patterns and payloads too, not some trivial endpoint. Then there's coordinated omission, where the tool waits for a slow response before sending the next request. That hides the true tail latency since the delayed requests never get counted, so you use tools that account for it. There's also open versus closed models, open means requests arrive at a fixed rate regardless of responses, closed means a fixed number of clients each wait for a response. Open is closer to real traffic, I think. Report percentiles since averages hide the tail, find the point where the service degrades, and test somewhere close to production so the results transfer.