Explaining Meltdown with parallel worlds, libraries, and a bank heist

Security vulnerabilities are always tricky to explain. People are supposed to be scared of them, because their data could be compromised, but the very nature of security problems means they involve obscure technical details that we usually don’t have to think about at all.
So, enter the analogy. A number of fun, strange, possibly useful analogies have sprung up this week to try to explain the major Meltdown and Spectre vulnerabilities that Google has discovered. Like any analogy, they fal