I once built a system for a government client. They were terrified of it crashing under heavy load, so the contract required that the site handle a load of 400,

I once built a system for a government client. They were terrified of it crashing under heavy load, so the contract required that the site handle a load of 400,000 simultaneous requests, and also that we demonstrate (through a 3rd party load testing company) that it could handle the load. Building it to handle the load was actually really easy (the architecture naturally handled many simultaneous requests easily), but testing it was very expensive, because it also involved making external requests we had to pay for. When the site finally launched, it did indeed get "a lot" of traffic. At its absolute peak, it hit around 20 requests per second. Sigh.