Okay, so I hunted this bug down and it was in my code, an interesting one. Sharing it here for amusement. The endpoint returning the string of "Looks like you'

Okay, so I hunted this bug down and it was in my code, an interesting one. Sharing it here for amusement. The endpoint returning the string of "Looks like you're based in Netherlands" is a nodej.js script that has this defined in the top: const MESSAGE = "Hey - it looks like you're based in $countryName." Sometime later in the code, in a function I manipulate this string in the module's export function: module.exports.getDiscountData = function(countryCode, countyName, etc) { // Fetch the right message let message = counryValues[countryCode]; // We get MESSAGE message = MESSAGE.replace("$countryName", countryName) // oops - we overwrote MESSAGE with a new value! message = MESSAGE.replace("$otherVariable", etc) return message; } And for each country, I have keys /values listed, like: "GB": MESSAGE, "NL": MESSAGE, "HU": MESSAGE_2 The problem was that once the first request replaced the country name, MESSAGE was mutated, and it now had the value "Hey - it looks like you're based in Netherlands." instead of "Hey - it looks like you're based in $countryName." I had no idea this would happen. And that a "const" in Javascript is a mutable thing. I fixed it by deep copying the string of the original message. The super interesting part is that I did not notice this problem, as when I was testing, I did low frequency testing. As I am using Lambda, I probably got a new container created, where MESSAGE was re-initialized. But with the HN traffic, the containers stayed alive longer, making this mutated string issue a problem.