1) you cannot have a runbook for everything, and even if you have a runbook you the best you could have found in this case is that something weird was happening in the VM. The setting had an insanely big value but it was accepted by the kernel, so you would assume it was a valid one. 2) the customer provided a huge amount of details, but it is usually very hard to explain what did you change from the base image. Most customers might not be willing to provide the full spec of their running system, as they might contain information they don't want to disclose. It is easier when the issue starts right after a change has been made, but this was not the case.