[titzer]

Joel of course hits the nail on the head about the two major things that cause abstractions to fall apart: performance and bugs (or debugging). In programming languages we talk about abstractions all the time--PL of course is all about abstractions. A computational abstraction like a bytecode, source language, or even machine code, can be proven be a proper (or full) abstraction, meaning there is no way for implementation details to leak in--you cannot observe the electrons flowing by executing A+B, after all.

...until you start measuring sidechannels, or the CPU or compiler has a bug.

I think about this a lot when dealing with VMs; a complex VM cannot hide its complexity when programs care about execution time, or when the VM actually has a bug.